Ignore pattern validation

[JustAnotherArchivist]

Using !ignore with a pattern that is not a valid Python regex should return an error instead of adding the broken pattern to the job config. Cf. #596

This was previously discussed on IRC at least 7-8 years ago but never implemented (nor apparently filed as an issue). One complication is that Python's regexes change over time. For example, (?i) to enable case-insensitive matching used to be valid anywhere in the pattern, but Python 3.11 and up only allows it at the beginning. So this validation would need to take the pipeline's Python version into account.