[BUGFIX] Return InaccessibleFolder for non existing processed folder

[BUGFIX] Return InaccessibleFolder for non existing processed folder

When the processed folder of a folder isn't present the ResourceStorage
tries to create it. When storage is offline or write-only this will fail
and as a result you run into exceptions on several places in BE.

This change makes sure there is a InaccessibleFolder object returned
in these cases.

Resolves: #61688
Releases: master, 6.2
Change-Id: Ie32c09f8d322914234860903c18ca44ff5cbf5db
Reviewed-on: http://review.typo3.org/36397
Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>