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.

Backport of https://github.com/owncloud/core/pull/11992 to stable7

@DeepDiver1975 @craigpg @icewind1991 

Before we accept this backport we need to test whether that really fixes https://github.com/owncloud/core/issues/11869 (CC @karlitschek)