Person edit: a failed save shows no error #2268

When saving an edited person fails, the edit form shows nothing: no error, no banner, and the unsaved changes stay on screen as if they were pending.

What is wrong. The handler for the person PATCH records the request as a success whatever the response was. The failure is reported to Rollbar, but the form only shows the backend error for a failed request, so the nurse is never told. Registering a new person does not have this problem, because its handler stores the real result.