[BUGFIX] Unreadable directories must not crash filemount storages

If a *single* directory in the fileadmin tree can't be read, the whole
select box of the file mounts is empty.

Just hide the entry which is not readable

Resolves: #63915
Releases: master, 6.2
Change-Id: I96d5472bb284c259b986b88a766b0fa1e6dfca07
Reviewed-on: http://review.typo3.org/35512
Reviewed-by: Alexander Opitz <opitz.alexander@googlemail.com>
Tested-by: Alexander Opitz <opitz.alexander@googlemail.com>