ceph.in: Re-enable ceph interactive mode (missing its output).

Also, loop on error.  There's no reason to exit the interpreter loop on
an error, and it's probably less annoying if we don't.  Print the error,
and any output, and continue.

Fixes: #5746
Signed-off-by: Dan Mick <dan.mick@inktank.com>
Merge pull request #487 from ceph/wip-5746

ceph.in: Re-enable ceph interactive mode (missing its output).

Reviewed-by: Sage Weil <sage@inktank.com>