Properly catch 503 storage not available in getQuotaInfo

When doing a PROPFIND on the root and one of the mount points is not
available, the returned quota attributes will now be zero.

This fix prevents the expected exception to make the whole call fail.
Merge pull request #11791 from owncloud/webdav-getquotainfo-503-handling

Properly catch 503 storage not available in getQuotaInfo