User feedback: report network issues to the user

Currently if a network issue occurs (like returning a 407 on a POST request) nothing will be reported to the user, causing unexpected results.

For example, say a user edits a title field. Then the user clicks on save. This will first execute a POST request saving the title field to the server and then execute a GET request saving the JCR session. If the first POST request fails because of a network issue (like 404/405/407/etc) and the second GET request is handled successfully, the user will expect the title field to be saved, as there is no warning shown that something went wrong, but instead it is not saved.