I commented on the wrong line: New exceptions are breaking changes. Someone may be catching a `ValueError` or `AttributeError` already in anticipation of a certain kind of exception. Replacing those with a new (ostensibly improved) Exception is a breaking API change and is backwards incompatible.

Certainly you're adding something but you're also removing old behaviour. I doubt people "depend" on that but instead have already worked around that. We cannot fit that into 2.3.