Raise an error if the converter arguments cannot be parsed

[pgjones]

This could happen for example with min=0;max=500 as the ; is not a word character everything before it is ignored in the regex during the finditer call. This then lefts the user confused as the min=0; was silently ignored.

fixes #2822

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.