If a not existent cache group is specified
to the flushCachesInGroup methods, there is no
indication that actually nothing happened.

Throw an exception instead, so that calling code
can easily be fixed.

Resolves: #58465
Releases: 6.2
Change-Id: I9617fcee9abfa27a9cb76a3fd12543c62420e719
Reviewed-on: https://review.typo3.org/26981
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Felix Oertel
Tested-by: Felix Oertel