ceph.in: Better error on bad arg to 'tell'

Previously would get a rather enigmatic error:
  UnboundLocalError: local variable 'ret' referenced before assignment

Now give something sensible:
  ceph_argparse.ArgumentValid: Bad target type 'mds'

Also update a couple of the other catch-all exception handlers
so that they will let the (nicer) ArgumentError exception through
for humans to see instead of munging them into RuntimeErrors.

Signed-off-by: John Spray <john.spray@inktank.com>