Fix Internal Server Error for Non-Local Users in Room Actions

[TrevisGordan]

Problem

see: #16924
The [POST /_matrix/client/v3/rooms/{roomId}/-action-] endpoint currently returns an Internal Server Error when processing a payload containing a user_id from a different home server or an invalid (non-local) user ID with non existing room. This occurs due to an unhandled standard Exception in the get_local_current_membership_for_user_in_room function within roommember.py.

This PR addresses the issue by improving the error handling for non-local user IDs.

1. Modified Exception Handling: Replace the standard Exception in get_local_current_membership_for_user_in_room with a SynapseError, providing a clear and specific error message to the API consumer.

   message = f"Provided user_id {user_id} is a non-local user"
   raise SynapseError(HTTPStatus.BAD_REQUEST, message, errcode=Codes.M_BAD_JSON)

Testing

• Added unit tests to verify correct error handling for non-local user IDs.

Notes

Closes: #16924

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct
  (run the linters)

[anoadragon453]

Some minor wording changes, but otherwise LGTM. Thanks!