Improve error message for bad indexed assignment

[JukkaL]

The error message generated for this fragment is not very informative:

from typing import Optional, List

def f(d: Optional[List[int]]) -> None:
    d[0] = 1  # Unsupported target for indexed assignment

It would be better to include the type in the message, such as like this:

t.py:4: error: Unsupported target for indexed assignment ("Optional[List[int]]")

#2900 is related, but this issue is more general.

[ndjman7]

May I try this? I want to this issue. @JukkaL

[JukkaL]

@ndjman7 Sure, go ahead!

[akerami]

@ndjman7 how is your progress on this? Are you still working on it?

[ndjman7]

@akerami sure. I'm working on it.

[ndjman7]

@JukkaL Please review the code.