ceph_argparse.py: clean up error reporting when required param missing

Treat "need 1, got 0" as a special case, and change the message to
"missing required parameter <x>".  Also, when failing for that reason,
print the command concise description and its helptext.

Fixes: #6384
Signed-off-by: Dan Mick <dan.mick@inktank.com>
Merge pull request #666 from dmick/wip-6384

ceph.in: clean up error message when missing required parameter