mon: set ceph osd (down|out|in|rm) error code on failure

Instead of always returning true, the error code is set if at least one
operation fails.

EINVAL if the OSD id is invalid (osd.foobar for instance).
ENOENT if the OSD does not exist
EBUSY if trying to remove and OSD that is up.

When used with the ceph command line, it looks like this:

    ceph -c ceph.conf osd rm osd.9
    Error ENOENT: osd.9 does not exist.
    ceph -c ceph.conf osd rm osd.0
    Error EBUSY: osd.0 is still up; must be down before removal.
    kill PID_OF_osd.0
    ceph -c ceph.conf osd down osd.0
    marked down osd.0.
    ceph -c ceph.conf osd rm osd.0 osd.1
    Error EBUSY: removed osd.0, osd.1 is still up; must be down before removal.

http://tracker.ceph.com/issues/6824 fixes #6824

Signed-off-by: Loic Dachary <loic@dachary.org>