[BUGFIX] Clean up error handler for site handling

This patch handles several issues related to the new
site error handling introduced in TYPO3 v9.2.0:

1. It adds unit tests
2. It adds a missing exception when an error handler
is configured which does not implement the PageErrorHandlerInterface
3. It fixes one minor issue in PageContentErrorHandler
where a wrong "InvalidArgumentException" was thrown
4. All PageErrorHandler logic was moved from EXT:frontend
to EXT:core, as this would be a penalty across packages -
the "Site" entity depends on them, so they must go to the same
package (core).

Releases: master
Resolves: #85101
Change-Id: Ibdc05024abd7c719dd8d5dcb3388bf1679b69990
Reviewed-on: https://review.typo3.org/57057


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>