gluster: raise suitable exception based on errno for peerDetach

Raising suitable exception based on the errno given by gluster
cli output for verb peerDetach.

Change-Id: I557a264445806695bb83d77400993ef9c8cfa698
Signed-off-by: ndarshan <dnarayan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22284
Reviewed-by: Bala.FA <barumuga@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>