Editor shows 'saved' even if the server has an error in saving edits

The editor shows 'saved' when it receives any response to its 'POST' request, for example server error 500.

We should show an 'error' if we receive 4xx or 5xx responses, and potentially 1xx or 3xx.

Start by locating the editor code that handles the POST response for saving edits. Check how response status codes currently determine the displayed save state. Done means 4xx and 5xx responses show an error instead of “saved”; the issue also suggests deciding how 1xx and 3xx responses should behave.

Written by the indexing model from the issue text.