REST API: Attempting to create or update a non-existent setting doesn't return an error response

[johnbillion]

Attempting to create a new setting via a POST request to /wp/v2/settings returns a 200 response, despite the request having been invalid. The same goes for attempting to update a non-existent setting with a PUT request.

I would have expected maybe a 400 from such requests.

The internal logic in WP_REST_Settings_Controller routes the request through the update_item() method, treats it as a setting update, and silently ignores the problem.

[johnbillion]

The test in 41604.test.patch​ demonstrates the issue.