Fix #1589: auth_handler: raise exception on init_request error

Fix #1589: auth_handler: raise exception on init_request error

cfs_* methods cann now die (rightfully so) when the IPCC endpoint is
not connected, or another grave IPCC error arised.

As we did not catch those problems in the RPCEnvironments
init_request method, which loads the user config, this got
propagated to the anyevents auth_handler call in its
unshift_read_header method where then all errors where processed in
the same way => with an unauthorized response logging an logged in
user out.

So catch this error and raise an internal server errror exception
instead. Anyevent needs some minor modifiaction in a separate patch
to handle PVE::Exceptions correctly, so this is the partial fix for
bug #1589

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