Don't overwrite UserVisibleError when updating selectors fails

[jhrozek]

Summary

We would return a UserVisibleError when updating selectors, but then
overwrite it in the caller who caled the validation, which resulted in
the server returning a 500 HTTP code to the client.

Change Type

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

curl -XPUT -vv -H "Authorization: Bearer $MINDER_BEARER_TOKEN" 'http://localhost:8080/api/v1/profile' --data-binary @/Users/jakub/devel/playground/selectors/repro.json

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.