Better errors from extends

What does this MR do?

This MR closes #63135 (moved) and will also fix #51846 (moved). We're not going to include line numbers because of the complexity involved - it's quite easy to include the context of the configuration error which is still useful for debugging.

The backend change replaces the plaintext Exception error message with a formatted JSON string capable of returning multiple errors with details.

The frontend renders the JSON string as given and puts the validation results message inside of a callout. This will help with #62930 (moved).

before	after

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers