Nicer error messages for Union and Optional types

[FuegoFro]

The error message you get when something is Optional and you try to access it isn't as friendly as it might be (with --strict-optional). Eg if I have

def foo(s: Optional[str]) -> None:
    print(s.count('a'))

It says error: Some element of union has no attribute "count".

It would be nicer if it said something like Variable 's' may be None or something a bit special cased to the Union with None case.

Generalizing a bit, one could imagine better handling of Unions in general by specifying what is in the Union or even which types in the Unions are causing the issue.

[Nurdok]

I'm looking at working on this.