Ceph

The documentation for "ceph osd purge" does not contain a lot of information about the command and its expectations. In
particular, the meaning of what is meant by this message is unclear.

In response to the command "ceph osd purge 1 --yes-i-really-mean-it", we get:
2018-05-10 15:18:03.444 7f29c0ae2700 2 mon.a@0(leader) e1
send_reply 0x556281707d40 0x5562818cdcc0 mon_command_ack([{"prefix": "osd purge", "sure": "--yes-i-really-mean-it", "id": 0}]=-16 osd.1 is not `down`.
v16) v1

...notice that "down" is in quotes, suggesting it has a special meaning. The command requires that the OSD process not be running, and this is what
is meant by "down". However, it's entirely reasonable for a user to think that it means the process should be /marked/ down, eg. with the "osd down"
command, which does not satisfy the requirement.

At least two things would improve this:
1) a brief mention that "osd purge" requires the OSD process to not be running in the documentation;
2) change the response to something more explicit and/or suggestive of the solution, perhaps along the lines of "cannot purge, osd.1 process [PID] is still running"

It also may be worth considering a couple of new features for the "ceph" command:
1) consistently print user-facing console messages, especially on failure (ie. print the above message to the user rather than requiring them to go to the logfile), provide
an option --quiet to disable this for scripting;
2) print helpful suggestions, similar to gcc and clang: "cannot purge... Did you try killing process [PID] first?"