1.2.1: Do not migrate when a replicaset has no leader

Fixed

• When a replicaset has no healthy leader candidate (its leader
  is disabled / down / not yet elected during stateful failover), migrator.up()
  used to silently skip it and report success. Make sure every replicaset has
  a healthy leader candidate before migrating. If a replicaset has no active
  leader yet or its leader is not healthy, fail so the caller can wait
  for the leader and retry.