better handle errors raised while applying filterwarnings (#7864)

[charlesaracil-ulti]

closes #7864

Add a more explicit error when raising exception while trying to import filterwarnings.

[bluetech]

Thanks for working on this @charlesaracil-ulti.

Some comments:

1. It would be good if the error says which warning filter causes the problem.

2. The function parse_warning_filter can fail in other places than just the warnings._getcategory call. It would be good to handle all sort of invalid warning filters while we're at it.

3. Right now the exception would be raised "lazily", i.e. only is something actually triggers it -- which might be a config-time warning, or in the likely case that there isn't one, during the first test setup. We should maybe consider doing a "pre-check" so it's always triggered consistently and then can be assumed to be valid. Though it will slow down startup a tiny bit...

4. It would be nice (though not absolutely necessary) to format the exception a little more nicely, like a ConftestImportFailure is formatted for example. Might not be worth the effort though...

When I get some time I'll try to provide some pointers if needed.

[charlesaracil-ulti]

Thanks @bluetech!
I'll be able to work on that next Friday, although I'm quite new to this project and I'll be happy to have some pointers anyway :)

[charlesaracil-ulti]

Didn't have as many time today as I expected, still addressed your 1. to get one thing done, will get back to it asap