[BUGFIX] T3editor: Make errors/exceptions show correctly

* Display T3editor.lang.errorWhileSaving in a flashmessage.
* Pass exception-messages through the AJAX-call and display
  them in the flashmessage as well.

Change-Id: Ia743b5a3fc9a7b272854920bcc0e9d081d4822ef
Resolves: #53115
Releases: 6.2, 6.1, 6.0, 4.5
Reviewed-on: https://review.typo3.org/25080
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters