[BUGFIX] Superfluous usage of ObjectManagerException

The workspace backend module uses
\TYPO3\CMS\Extbase\Object\ObjectManagerException instead of
\TYPO3\CMS\Extbase\Object\ObjectManager.

Change-Id: Ieb30ed6cd3e8d33de7270800044b0eeebf83be90
Fixes: #52824
Releases: 6.0, 6.1, 6.2
Reviewed-on: https://review.typo3.org/24775
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader