Fixed error message formatting in rebalance API: previously they could be displayed as unclear dumps of internal Erlang structures.

Added `wait_health_check` option to node evacuation CLI and API. This is a time interval when the node reports "unhealthy status" without beginning actual evacuation. We need this to allow a Load Balancer (if any) to remove the evacuated node from balancing and not forward (re)connecting clients to the evacuated node.