ReplicatedPG: CEPH_OSD_OP_WATCH return -ENOENT if !obs.exists

Fixes: #7671
Signed-off-by: Samuel Just <sam.just@inktank.com>
test/system/st_rados_watch: expect ENOENT for watch on non-existent object

Signed-off-by: Samuel Just <sam.just@inktank.com>
test/librados/watch_notify: create foo before watching

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1441 from ceph/wip-7671

Wip 7671

Reviewed-by: Sage Weil <sage@inktank.com>