Give more detail about original definition when override is incompatible

[JukkaL]

If a method override is incompatible with the definition in a base class, it would be helpful to give the file + line number of the base class definition. Maybe we should also print out the signature of the original definition if we can't pinpoint the error to an incompatible argument/return type (that is, when we give the generic Signature of "x" incompatible with supertype "A" error).

Follow-up to #5025.

[ilevkivskyi]

I think just printing the signature would be better IMO. In other places (bad keyword argument IIRC) we put notes "blah" defined here, but they are not always clear, and previously caused troubles in incremental mode and/or if output is sorted by file.

[JukkaL]

The sounds reasonable. Alternatively, we could implement a more robust way of pointing to lines elsewhere in the target program.

[hauntsaninja]

I think we have better error messages here now.