ceph: return error code when failing to get result from admin socket

Make sure we return a non-zero result code when we fail to read something
from the admin socket.

Backport: cuttlefish, bobtail
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 393c9372f82ef37fc6497dd46fc453507a463d42)