[Backport to 7.9] - Error messages in document editor disappear automatically when a ping (ajax call) arrives

When illegal data is entered into a document field in the CMS and a document is saved, an error message in red is shown at the top. Sometimes this error message disappears automatically after 1 second or so, but sometimes the error message stays there until the document is refreshed (e.g. because a Xinha field is clicked, or tab is used to jump to the next field).

Steps to reproduce:
1. In the CMS test, create a new Event document
2. Leave the mandatory title field empty and click 'save'

An error message is shown "Required field missing". This message suddenly disappears when the "Introduction" or "Body" field is clicked. It also disappears when you first click the "Title" field and then press tab.

3. Enter a title again and click "save"
4. In the "Date" field, enter the (illegal) hour "24" and click save

An error message "24 is not between 0 and 23" will be shown very briefly, and then disappears automatically. The date field is reset to the old value. Users are likely to miss such an error message and wonder what went wrong.

Expected behavior: all error messages remain visible until their value is corrected
Actual behavior: error messages sometimes 'suddenly' disappear