ceph-mon: be a bit more verbose on error

Motivated by #7489

Signed-off-by: Sage Weil <sage@inktank.com>
mon: only do timecheck with known monmap

If we are still on monmap epoch 0, our mon ranks cannot yet be trusted
since there is not yet a shared source of truth from paxos.  If we do
timechecks, the code gets confused about the ranks in e.g. the
timecheck_waiting map.

Fixes: #7692
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1461 from ceph/wip-7692

mon: on timecheck on monmap 0

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>