Generate better error message for incompatible **kwargs

This output is from #8772:

$ mypy test.py 
test.py:6: error: Argument 2 to "dumps" has incompatible type "**Dict[str, int]"; expected "bool"
test.py:6: error: Argument 2 to "dumps" has incompatible type "**Dict[str, int]"; expected "Optional[Type[JSONEncoder]]"
test.py:6: error: Argument 2 to "dumps" has incompatible type "**Dict[str, int]"; expected "Optional[Tuple[str, str]]"
test.py:6: error: Argument 2 to "dumps" has incompatible type "**Dict[str, int]"; expected "Optional[Callable[[Any], Any]]"
Found 4 errors in 1 file (checked 1 source file)

The error messages are pretty confusing. We should special case this and produce a customized error message that explains the situation better.

Here's the code that triggers the errors:

import json

json_kwargs = dict(indent=2)

json.dumps({}, **json_kwargs)

Some ideas about what to do:

• Generate a single error message instead of multiple ones.
• Add a note suggesting the use of Dict[str, Any] or a TypedDict type for the **kwargs argument.
• Maybe also explain that mypy matches the value type against all arguments, or add a link to documentation (add something to common issues, for example).

Inizia riproducendo l’esempio in test.py con json.dumps e confrontalo con l’output di #8772. Traccia la segnalazione degli errori esistente per **kwargs, quindi fai in modo che il risultato sia un’unica diagnosi chiara con indicazioni su Dict[str, Any] o TypedDict e verifica che gli errori ripetuti e confusi siano scomparsi.