[FEATURE] Fetch error pages via subrequests instead of HTTP

There are many reasons to not issue a second HTTP request for error pages
and instead issue an in-process subrequest.  This patch makes that change,
and refactors PageContentErrorHandler a bit while there.

Resolves: #94402
Releases: master
Change-Id: I6da9d753bebc0ef038278185629433587a9b7162
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69547


Tested-by: Benjamin Franzke <bfr@qbus.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Benni Mack <benni@typo3.org>