ceph.in: better error message when daemon command returns nothing

Fixes: #5683
signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
ceph_rest_api.py: remove unused imports

Fixes: #5684
Signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>