[TASK] Cleanup error handling code and settings description

The error handling code for PHP errors needs some cleanup.
So does the description of the related settings in the Install Tool.

Resolves: #61235
Releases: 6.3, 6.2
Change-Id: Ibd95fcdaa6a50a870035037a2332b95d2c5e1266
Reviewed-on: http://review.typo3.org/32568
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>