fix(checks): avoid crash when check flag cannot be parsed

[nijel]

Instead make the check failing with a proper description.

Fixes #17816

[codecov]

Codecov Report

❌ Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.99%. Comparing base (ed919ee) to head (7089143).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
weblate/checks/base.py	90.90%	1 Missing ⚠️

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