Clean up NotifierNotFound error causing noise in the test suite

[aaronayres35]

Problem Description
See comment: https://github.com/enthought/chaco/pull/591/files#r606044010

In #591 a change was made that introduces a very noisy error message in the test suite (no tests fail though). The causing change was to code that was already clearly broken (looked like a copy paste error) and seems to generally be an improvement, but something is still incorrect. We try to unhook a listener before it has been unhooked because the magically named static handler is called with both old and new !=None on the first change. Thus, the old object never had its listener hooked up in the first place. This issue is about avoiding this error here. However, I will open a separate issue about the general use of this pattern.

For ref here is what you see in the test suite:

Details

.Exception occurred in traits notification handler for object: <chaco.tools.range_selection.RangeSelection object at 0x7fb94d99c9e8>, trait: axis, old value: index, new value: value
Traceback (most recent call last):
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/trait_notifiers.py", line 342, in __call__
    self.handler(*args)
  File "/Users/aayres/Desktop/chaco/chaco/tools/range_selection.py", line 669, in _axis_changed
    self.__mapper_changed, old + "_mapper", remove=True
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/has_traits.py", line 2330, in observe
    dispatcher=_ObserverDispatchers[dispatch],
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/observe.py", line 59, in observe
    remove=remove,
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 54, in add_or_remove_notifiers
    callable_()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 94, in __call__
    step()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 118, in _add_or_remove_extra_graphs
    remove=self.remove,
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 54, in add_or_remove_notifiers
    callable_()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 94, in __call__
    step()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 150, in _add_or_remove_maintainers
    change_notifier.remove_from(observable)
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observer_change_notifier.py", line 123, in remove_from
    raise NotifierNotFound("Notifier not found.")
traits.observation._exceptions.NotifierNotFound: Notifier not found.
Exception occurred in traits notification handler for object: <chaco.tools.range_selection.RangeSelection object at 0x7fb94b547e60>, trait: axis, old value: index, new value: value
Traceback (most recent call last):
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/trait_notifiers.py", line 342, in __call__
    self.handler(*args)
  File "/Users/aayres/Desktop/chaco/chaco/tools/range_selection.py", line 669, in _axis_changed
    self.__mapper_changed, old + "_mapper", remove=True
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/has_traits.py", line 2330, in observe
    dispatcher=_ObserverDispatchers[dispatch],
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/observe.py", line 59, in observe
    remove=remove,
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 54, in add_or_remove_notifiers
    callable_()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 94, in __call__
    step()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 118, in _add_or_remove_extra_graphs
    remove=self.remove,
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 54, in add_or_remove_notifiers
    callable_()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 94, in __call__
    step()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 150, in _add_or_remove_maintainers
    change_notifier.remove_from(observable)
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observer_change_notifier.py", line 123, in remove_from
    raise NotifierNotFound("Notifier not found.")
traits.observation._exceptions.NotifierNotFound: Notifier not found.
Exception occurred in traits notification handler for object: <chaco.tools.range_selection.RangeSelection object at 0x7fb94dc987d8>, trait: axis, old value: index, new value: value
Traceback (most recent call last):
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/trait_notifiers.py", line 342, in __call__
    self.handler(*args)
  File "/Users/aayres/Desktop/chaco/chaco/tools/range_selection.py", line 669, in _axis_changed
    self.__mapper_changed, old + "_mapper", remove=True
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/has_traits.py", line 2330, in observe
    dispatcher=_ObserverDispatchers[dispatch],
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/observe.py", line 59, in observe
    remove=remove,
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 54, in add_or_remove_notifiers
    callable_()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 94, in __call__
    step()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 118, in _add_or_remove_extra_graphs
    remove=self.remove,
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 54, in add_or_remove_notifiers
    callable_()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 94, in __call__
    step()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 150, in _add_or_remove_maintainers
    change_notifier.remove_from(observable)
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observer_change_notifier.py", line 123, in remove_from
    raise NotifierNotFound("Notifier not found.")
traits.observation._exceptions.NotifierNotFound: Notifier not found.
Exception occurred in traits notification handler for object: <chaco.tools.range_selection.RangeSelection object at 0x7fb94dce7728>, trait: axis, old value: index, new value: value
Traceback (most recent call last):
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/trait_notifiers.py", line 342, in __call__
    self.handler(*args)
  File "/Users/aayres/Desktop/chaco/chaco/tools/range_selection.py", line 669, in _axis_changed
    self.__mapper_changed, old + "_mapper", remove=True
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/has_traits.py", line 2330, in observe
    dispatcher=_ObserverDispatchers[dispatch],
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/observe.py", line 59, in observe
    remove=remove,
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 54, in add_or_remove_notifiers
    callable_()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 94, in __call__
    step()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 118, in _add_or_remove_extra_graphs
    remove=self.remove,
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 54, in add_or_remove_notifiers
    callable_()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 94, in __call__
    step()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 150, in _add_or_remove_maintainers
    change_notifier.remove_from(observable)
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observer_change_notifier.py", line 123, in remove_from
    raise NotifierNotFound("Notifier not found.")
traits.observation._exceptions.NotifierNotFound: Notifier not found.
Exception occurred in traits notification handler for object: <chaco.tools.range_selection.RangeSelection object at 0x7fb94dd21b48>, trait: axis, old value: index, new value: value
Traceback (most recent call last):
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/trait_notifiers.py", line 342, in __call__
    self.handler(*args)
  File "/Users/aayres/Desktop/chaco/chaco/tools/range_selection.py", line 669, in _axis_changed
    self.__mapper_changed, old + "_mapper", remove=True
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/has_traits.py", line 2330, in observe
    dispatcher=_ObserverDispatchers[dispatch],
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/observe.py", line 59, in observe
    remove=remove,
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 54, in add_or_remove_notifiers
    callable_()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 94, in __call__
    step()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 118, in _add_or_remove_extra_graphs
    remove=self.remove,
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 54, in add_or_remove_notifiers
    callable_()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 94, in __call__
    step()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 150, in _add_or_remove_maintainers
    change_notifier.remove_from(observable)
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observer_change_notifier.py", line 123, in remove_from
    raise NotifierNotFound("Notifier not found.")
traits.observation._exceptions.NotifierNotFound: Notifier not found.
Exception occurred in traits notification handler for object: <chaco.tools.range_selection.RangeSelection object at 0x7fb94dd4df10>, trait: axis, old value: index, new value: value
Traceback (most recent call last):
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/trait_notifiers.py", line 342, in __call__
    self.handler(*args)
  File "/Users/aayres/Desktop/chaco/chaco/tools/range_selection.py", line 669, in _axis_changed
    self.__mapper_changed, old + "_mapper", remove=True
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/has_traits.py", line 2330, in observe
    dispatcher=_ObserverDispatchers[dispatch],
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/observe.py", line 59, in observe
    remove=remove,
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 54, in add_or_remove_notifiers
    callable_()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 94, in __call__
    step()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 118, in _add_or_remove_extra_graphs
    remove=self.remove,
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 54, in add_or_remove_notifiers
    callable_()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 94, in __call__
    step()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 150, in _add_or_remove_maintainers
    change_notifier.remove_from(observable)
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observer_change_notifier.py", line 123, in remove_from
    raise NotifierNotFound("Notifier not found.")
traits.observation._exceptions.NotifierNotFound: Notifier not found.
Exception occurred in traits notification handler for object: <chaco.tools.range_selection.RangeSelection object at 0x7fb94b547b48>, trait: axis, old value: index, new value: value
Traceback (most recent call last):
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/trait_notifiers.py", line 342, in __call__
    self.handler(*args)
  File "/Users/aayres/Desktop/chaco/chaco/tools/range_selection.py", line 669, in _axis_changed
    self.__mapper_changed, old + "_mapper", remove=True
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/has_traits.py", line 2330, in observe
    dispatcher=_ObserverDispatchers[dispatch],
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/observe.py", line 59, in observe
    remove=remove,
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 54, in add_or_remove_notifiers
    callable_()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 94, in __call__
    step()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 118, in _add_or_remove_extra_graphs
    remove=self.remove,
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 54, in add_or_remove_notifiers
    callable_()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 94, in __call__
    step()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 150, in _add_or_remove_maintainers
    change_notifier.remove_from(observable)
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observer_change_notifier.py", line 123, in remove_from
    raise NotifierNotFound("Notifier not found.")
traits.observation._exceptions.NotifierNotFound: Notifier not found.
Exception occurred in traits notification handler for object: <chaco.tools.range_selection.RangeSelection object at 0x7fb94b547d58>, trait: axis, old value: index, new value: value
Traceback (most recent call last):
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/trait_notifiers.py", line 342, in __call__
    self.handler(*args)
  File "/Users/aayres/Desktop/chaco/chaco/tools/range_selection.py", line 669, in _axis_changed
    self.__mapper_changed, old + "_mapper", remove=True
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/has_traits.py", line 2330, in observe
    dispatcher=_ObserverDispatchers[dispatch],
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/observe.py", line 59, in observe
    remove=remove,
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 54, in add_or_remove_notifiers
    callable_()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 94, in __call__
    step()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 118, in _add_or_remove_extra_graphs
    remove=self.remove,
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 54, in add_or_remove_notifiers
    callable_()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 94, in __call__
    step()
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observe.py", line 150, in _add_or_remove_maintainers
    change_notifier.remove_from(observable)
  File "/Users/aayres/.edm/envs/chaco-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_observer_change_notifier.py", line 123, in remove_from
    raise NotifierNotFound("Notifier not found.")
traits.observation._exceptions.NotifierNotFound: Notifier not found.
...........
----------------------------------------------------------------------
Ran 549 tests in 5.399s

OK (skipped=8)

Reproduction Steps:
Run the test suite on master now that #591 has been merged

Expected behavior:

Don't see the warnings in the test suite, and handlers get hooked up / removed at the appropriate times