ceph: error out properly when failing to get commands

If we make ret positive here we miss the failure check below.  Instead,
just set outs appropriately.

Signed-off-by: Sage Weil <sage@inktank.com>
ceph: fix return code for multi-target commands

Signed-off-by: Sage Weil <sage@inktank.com>
ceph: remove space when prefix is blank

Signed-off-by: Sage Weil <sage@inktank.com>