code: public void fail()
      {
          logger.error("Failed creating a merkle tree for {}, {} (see log for details)", desc, initiator);
          // send fail message only to nodes >= version 2.0
          MessagingService.instance().sendOneWay(new ValidationComplete(desc).createMessage(), initiator);
      }
docs: Called when some error during the validation happened.
  This sends RepairStatus to inform the initiator that the validation has failed.
  The actual reason for failure should be looked up in the log of the host calling this function.