Give better error message if type inference fails because of unsatisfiable upper bound

[ilevkivskyi]

After #5699 in certain cases (see testWideOuterContextEmptyError) if type inference fails, mypy gives an obscure error like

Incompatible types in assignment (expression has type "List[<nothing>]", variable has type "List[str]")

This error message could be improved. Here are some ideas:

• Record that we didn't use the outer context because of unsatisfied bound and add a note about this
• (Probably better) If type variable with a bound is used in an invariant context and we can't infer a value, we currently infer C[<nothing>]. What if we'd instead inferred C[<bound>]?