Reduce log ERROR to WARN to avoid startup and shutdown noise as replicas connect/disconnect

When using mitosis replication for ADS there is a period of time during start-up when one replica cannot connect to another - because separate machines rarely start at the same rate.

During this time, ClientConnectionManager logs ERRORs as MINA gets 'Connection refused' socket errors.

I'd like to propose that the replication service only logs WARN messages for this condition.