[TASK] Include PHP warnings as exceptional errors

Resolves: #56683
Releases: 6.2
Change-Id: I471e5b44eeb2dcb29217fbea3a33b4aba95e2cdd
Reviewed-on: https://review.typo3.org/28191
Reviewed-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Christian Kuhn
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring