Catch the exception thrown by AGG

[jwiggins]

Resolves #750

This is basically the same change as celiagg/celiagg#97, but with SWIG exception handling instead of Cython. A test has also been added to show that the error is now recoverable.

(Celiagg keeps its rasterizer instance around between draw calls, so it needed an additional change to the AGG code which is not needed here)

[rahulporuri]

LGTM

[rahulporuri]

I tested this branch against the code in enthought/chaco#474 and it works as expected. On the main branch, python just halted quietly (with some additional information when we use the -Xfaulthandler flag). On this branch, I get a much cleaner error -

Traceback (most recent call last):
  File "c:\users\rporuri\work\github\ets\enable\enable\qt4\base_window.py", line 243, in paintEvent
    self.handler.paintEvent(event)
  File "c:\users\rporuri\work\github\ets\enable\enable\qt4\base_window.py", line 78, in paintEvent
    self._enable_window._paint(event)
  File "c:\users\rporuri\work\github\ets\enable\enable\abstract_window.py", line 538, in _paint
    self.component.draw(gc, view_bounds=(0, 0, size[0], size[1]))
  File "c:\users\rporuri\work\github\ets\enable\enable\component.py", line 418, in draw
    self._draw(gc, view_bounds, mode)
  File "c:\users\rporuri\work\github\ets\enable\enable\component.py", line 799, in _draw
    self._dispatch_draw(layer, gc, view_bounds, mode)
  File "c:\users\rporuri\work\github\ets\enable\enable\container.py", line 280, in _dispatch_draw
    component._dispatch_draw(layer, gc, new_bounds, mode)
  File "c:\users\rporuri\work\github\ets\enable\enable\component.py", line 817, in _dispatch_draw
    handler(gc, view_bounds, mode)
  File "c:\users\rporuri\work\github\ets\chaco\chaco\base_xy_plot.py", line 463, in _draw_plot
    self._draw_component(gc, view_bounds, mode)
  File "c:\users\rporuri\work\github\ets\chaco\chaco\base_xy_plot.py", line 471, in _draw_component
    self._render(gc, pts)
  File "c:\users\rporuri\work\github\ets\chaco\chaco\lineplot.py", line 325, in _render
    render(gc, points, self.orientation)
  File "c:\users\rporuri\work\github\ets\chaco\chaco\lineplot.py", line 336, in _render_normal
    gc.stroke_path()
  File "c:\users\rporuri\work\github\ets\enable\kiva\agg\agg.py", line 1360, in stroke_path
    return _agg.GraphicsContextArray_stroke_path(self, *args)
OverflowError: Exceeded cell block limit

[jwiggins]

Drive-by fix. This caused a ResourceWarning when I was running the tests (which I hadn't seen before).

[jwiggins]

Thanks for the review/testing