mds: Table read failure should log at error level

Signed-off-by: John Spray <john.spray@inktank.com>
mds: Fix Dumper shutdown

It was shutting down messenger before monclient.  This
caused a sometimes-assertion when monclient tried to
reconnect in response to ms_handle_reset.

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