Improve UX during errors while parsing warning filters

[nicoddemus]

Fix #7864
Fix #9218

Closes #8343
Closes #7877

[nicoddemus]

Instead of an internal traceback on errors, we get a much nicer error message:

λ python -m pytest test_foo.py
ERROR: while parsing the following warning configuration:

  ignore::foo.MyWarning::-1

This error occurred:

foo.py:2: in <module>
    import invalid
E   ModuleNotFoundError: No module named 'invalid'

[RonnyPfannschmidt]

lovely 👍

[bluetech]

Much nicer!