Improve error message for overload overrides w/ swapped variants

[Michael0x2a]

This commit is intended to resolve #1270.

Currently, running mypy on the following program:

from typing import overload

class Parent:
    @overload
    def foo(self, x: A) -> A: ...
    @overload
    def foo(self, x: B) -> B: ...

class Child(Parent):
    @overload
    def foo(self, x: B) -> B: ...
    @overload
    def foo(self, x: A) -> A: ...

...will make mypy report the following error -- it considers the fact that we've swapped the order of the two variants to be unsafe:

test.pyi:10: error: Signature of "foo" incompatible with supertype "Parent"

This error message can be confusing for some users who may not be aware that the order in which your overloads are defined can sometimes matter/is something mypy relies on.

This commit modifies the error message to the following to try
and make this more clear:

test.pyi:10: error: Signature of "foo" incompatible with supertype "Parent"
test.pyi:10: note: Overload variants must be defined in the same order as they are in "Parent"

[Michael0x2a]

As a misc note, one idea the issue I linked to above mentioned was that we could maybe print out the signature of the parent method. I decided not to do this for now because it felt inconsistent: we don't do this in other cases where the user incorrectly overrides a method.

If printing out the parent signature seems like a helpful thing to do in general, I'm happy to make a new PR w/ that change.

[ilevkivskyi]

Thanks! This additional note makes sense. Also I think it is right to only show in in obvious cases.