[BUGFIX] Possible error 500

If PHP warnings are turned into fatal errors, trying to save a
read-only file will lead to a error 500 during AJAX call.

Change-Id: I95e772bfd95134bb52a7dadb4ab117aa4a464f33
Reviewed-on: https://review.typo3.org/26636
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers