chore: simplify format flags handling

[nijel]

Move merging to a single location where errors can be handled consistently.

See #17816

[codecov]

Codecov Report

❌ Patch coverage is 91.42857% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.00%. Comparing base (ed919ee) to head (efa070a).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
weblate/formats/base.py	83.33%	3 Missing ⚠️

❌ Your patch status has failed because the patch coverage (91.42%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.

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