Exit status when config error #15850

When a config error is detected, mypy outputs a warning, but will still exit successfully, thus not being noticed in a CI run.

Example output:

.mypy.ini: [mypy]: enable_error_code: Invalid error code(s): unused-awaitable  # TODO(PY312): explicit-override
Success: no issues found in 9 source files

Expected:
A non-0 exit status if the configuration file is not correct.