Improve error message for chained comparisons

[AlexWaygood]

We currently have a very specific test for chained comparisons on lines 506-508:

flake8-pyi/pyi.py

Lines 506 to 508 in 34c17bd

	if len(node.comparators) != 1:
	# mypy doesn't support chained comparisons
	self.error(node, Y002)

However, we then give a generic error message:

"Y002 If test must be a simple comparison against sys.platform or sys.version_info"

If we split this off into its own error code, we could give a much more specific error message:

YXXX Chained comparisons are not supported by type-checkers

Thoughts?

[hauntsaninja]

I'm okay with the current behaviour. To me "simple comparison" is clear enough and I can't see a good reason for selectively disabling only one of the two error codes.

[AlexWaygood]

I can't see a good reason for selectively disabling only one of the two error codes.

Yes, you're right -- it probably doesn't deserve its own error code. In which case, maybe we could make the error message for that code more customisable?

It just seems a bit silly to me that there's a comment in the source code that gives more information about why the error's being emitted than the error message that's given.

[JelleZijlstra]

Yes, maybe we could give a different error message with the same code? Not sure if that goes against general flake8 conventions.

Anyway, I don't think this is too important as it's not something that happens commonly.