Extend type hints to cover exceptions

[Rosuav (Chris Angelico)]

@oscarbenjamin

An expected exception is one that you realistically should always consider like FileNotFoundError but an unexpected exception would be something like mylist[0] raising an IndexError. Analogously the Rust typechecker will allow compiling code that does myvec[0] from an empty list (and will then panic at runtime rather than returning a checked exception) but would force you to handle a checked exception if you tried to open a file.

This still ignores the fact that tne entire benefit of exceptions is that you DON’T have to catch them the moment you could cause them.

Can we please stop trying to turn exceptions into a bad-performance version of return values?