Improve error messages when comparing against objects vs strings

[stephenworsley]

A simple change to error messages to address #4898.

[rcomer]

Wow, thanks for the fast work @stephenworsley! I agree that trying to list out exactly what didn't match might be more trouble than it's worth, but I have a suggestion to be a bit more explicit for the coord case.

[rcomer]

Hi @stephenworsley, this looks good to me. Some very minor suggestions in line. I also wonder if it is worth updating the relevant tests to use assertRaisesRegex and check the message:

[Can't seem to find them for cube.coord !]

iris/lib/iris/tests/unit/cube/test_Cube.py

Lines 2670 to 2678 in d56824c

	def test_fail_get_cell_measure(self):
	with self.assertRaises(CellMeasureNotFoundError):
	_ = self.cube.cell_measure("notarea")
	def test_fail_get_cell_measures_obj(self):
	a_cell_measure = self.a_cell_measure.copy()
	a_cell_measure.units = "km2"
	with self.assertRaises(CellMeasureNotFoundError):
	_ = self.cube.cell_measure(a_cell_measure)

iris/lib/iris/tests/unit/cube/test_Cube.py

Lines 2612 to 2620 in d56824c

	def test_fail_get_ancillary_variables(self):
	with self.assertRaises(AncillaryVariableNotFoundError):
	self.cube.ancillary_variable("other_ancill_var")
	def test_fail_get_ancillary_variables_obj(self):
	ancillary_variable = self.ancill_var.copy()
	ancillary_variable.long_name = "Number of observations at site"
	with self.assertRaises(AncillaryVariableNotFoundError):
	self.cube.ancillary_variable(ancillary_variable)

[rcomer]

Thanks @stephenworsley. Just waiting for the CI...

[rcomer]

Seems docs are failing because https://docs.scipy.org/ is down, so the intersphinx mapping doesn't work. I'm inclined to merge anyway: the only doc change here is the whatsnew entry and it was fine before rebase.