[BUGFIX] Validate page error handler result to report wrong configuration

Resolves: #50186
Releases: master, 8.7
Change-Id: I082a2d48608d43856cd60076852a19aec8a21a7e
Reviewed-on: https://review.typo3.org/54494


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Henning Liebe <h.liebe@neusta.de>
Tested-by: Henning Liebe <h.liebe@neusta.de>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>