Use more precise context for invalid type argument errors

[brianschubert]

Fixes #12274

Uses the actual invalid type argument as the error context when possible.

Given:

# flags: --pretty --show-column-number
class Foo[S, T: int]: 
    pass

x: Foo[str, str]

Before:

main.py:3:4: error: Type argument "str" of "Foo" must be a subtype of "int"  [type-var]
    x: Foo[str, str]
       ^

After:

main.py:3:13: error: Type argument "str" of "Foo" must be a subtype of "int"  [type-var]
    x: Foo[str, str]
                ^

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

AutoSplit (https://github.com/Toufool/AutoSplit)
- typings/cv2/mat_wrapper/__init__.pyi:7:23: error: Type argument "float" of "ndarray" must be a subtype of "tuple[int, ...]"  [type-var]
+ typings/cv2/mat_wrapper/__init__.pyi:7:34: error: Type argument "float" of "ndarray" must be a subtype of "tuple[int, ...]"  [type-var]