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.

Conflicts:
	lib/private/connector/sabre/directory.php
	lib/private/connector/sabre/file.php
Merge pull request #11994 from owncloud/stable7-sabre-convertstoragenotavailableexception

[stable7] Convert StorageNotAvailableException to SabreDAV exception