[BUGFIX] Install tool stays open if ENABLE_INSTALL_TOOL is not writable

Recognize if the ENABLE_INSTALL_TOOL file deletion fails
and keep the install tool closed in this case.

Change-Id: I78e1990b6a0b66ef8804ede9bbfa3546b08d4067
Fixes: #29674
Releases: 4.7, 4.6, 4.5
Reviewed-on: http://review.typo3.org/4906
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn