[FIX] Corpus viewer: warn on bad regex

[SanchoSamba]

Issue

Fixes #1068

Description of changes

Instead of crashing, a warning message is thrown on bad regex.

Includes

• Code changes
• Tests
• Documentation

[markotoplak]

Please delete this line (the correct import is already few lines up). We need to import from AnyQt so that the add-on is compatible both with Qt 5 and 6.

[markotoplak]

Thanks for tackling this. I see two things that should be improved in this PR:

1. The new error-showing functionality has to be tested, as in writing a unit test that verifies that the error is shown.
2. The UX should be unified with how Orange usually does this. We tend to use widgets errors for stuff like this (see how read_file error works in owcorpus).

[ajdapretnar]

/rebase

[ajdapretnar]

/rebase