[BUGFIX] Vague error message in t3lib_div

If the web server cannot write to typo3temp/, a vague error
message may appear: "File not written to disk! Write
permission error in filesystem?". Fix the issue by hinting
the user where he should check permissions.

Change-Id: I5509461e6dd48b5a60963a478f0643309c889901
Resolves: #18545
Releases: 4.7, 4.6, 4.5
Reviewed-on: http://review.typo3.org/6853
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader
Reviewed-by: Oliver Klee
Reviewed-by: Mario Rimann
Reviewed-by: Philipp Gampe
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch
Reviewed-by: Andreas Wolf
Tested-by: Andreas Wolf