auth_handler: respond with passed error if we get a PVE::Exception

auth_handler: respond with passed error if we get a PVE::Exception

Allows to fix a problem where a logged in connected client was logged
out because we could not verify him for this call as the cluster
filesystem was unavailable.

If we get such a exception then use it for responding.
THis is save as no logged out client can get ever do anything where
login privileges are required and a logged in client cannot to
anything during the problematic period, but does not gets logged out.
Partail fix for #1589

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>