[BUGFIX] Avoid exception on non-existing error handler

When a site is configured, but has no error handler
for a specific error case (e.g. a site has no
configured 404 page), then the regular error handler
should kick in.

This also allows to enable some previously wrong
tests (expected!).

Resolves: #85819
Releases: master
Change-Id: Iefc0e23460de0389b66083da4e6d7aca12b82c59
Reviewed-on: https://review.typo3.org/57877


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Wolf <andreas.wolf@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>