Catch forbidden http status code

If you try to do something on a DAV mount (external or federated share)
that is not allowed. We should not mark the storage as not available but
just fail somewhat gracefully.

Now by catching this and just properly returning the operation will just
fail (and notify the user) which is already a lot better then marking
the storage as unavailable and doing boom.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>