Convert StorageNotAvailableException to SabreDAV exception

Convert \OCP\Files\StorageNotAvailableException to
\Sabre\DAV\Exception\ServiceUnavailable for every file/directory
operation happening inside of SabreDAV.

This is necessary to avoid having the exception bubble up to remote.php
which would return an exception page instead of an appropriate response.
Merge pull request #12072 from owncloud/sabre-convertstoragenotavailableexception-secondtry

Convert StorageNotAvailableException to SabreDAV exception