Surface selector check errors to handlers

[jhrozek]

Summary

This is a minimal patch to surface selector errors through profile
handlers to clients. It does not expose the structured data (e.g. which
line and which column did the error occur at). I started working on
that, but realized that that's a bigger endevor that ought to be
implemented separately.

Fixes: #3987

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

by calling the profile handlers, both grpc through the minder client and HTTP through curl:

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

[coveralls]

coverage: 54.652% (+0.2%) from 54.497%
when pulling ef6981b on jhrozek:selectors_err_msg_simple
into 95535e5 on stacklok:main.