Backend: Update json_schema_validator to return meaningful error messages

Discussion came up from the below thread -

The following discussion from !146333 (merged) should be addressed:

• @lma-git started a discussion: (+4 comments)

  Blocking:

  Is this really the error message? It's quite cryptic. Could we improve this? 🙏

Proposal

Probably the JSON schema validator should be updated to produce more precise/UX-friendly error messages.

This will fix the other cases where we validate the value of a keyword against a json.