More descriptive error message when validating secrets with any of the fallback webhooks fails

[jhrozek]

Summary

Previously we would have returned a nil error but also a nil payload on
exhausting all previous secrets in a loop which would later error out when it
couldn't be parsed. That means that this bug has no security implications (we
would just get a 500 instead of a 400), but let's fix it nonetheless.

Change Type

Mark the type of change your PR introduces:

• 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

manually + a unit test

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: 50.169% (+0.03%) from 50.144%
when pulling 6a8a876 on fallback_webhook_secret_fix
into e6ec651 on main.