librustc: Improve error message for incompatible trait method signatures.
auto merge of #13333 : Ryman/rust/improve_incompatible_type_error, r=alexcrichton

This can be a frustrating error message, ideally we should print the signature mismatch, but hinting that it's a trait incompatibility helps tracking root cause. Also beefed up the testcases for this.

Ideally we would print the signature mismatch in the error helper?