TYPO3 Core

This is an SVN patch request.

Type: Bugfix

BT:

Branches: trunk, 4.4

Problem:
With issue #21355 we un-hardcoded the cache frontend settings for core caches. This now leads to an error if own core caches are defined and the frontend setting is missing. t3lib_div::makeInstance will then get an empty string as frontend class and just spits error "Class not found", without more infromation on what exactly went wrong. This makes this issue hard to track down.

Solution:
Throw a more meaningful error message in t3lib_cache if frontend defintion is missing.

(issue imported from #M15937)