Avoid ONUs being set to unreachable after multiple unsuccessful OMCI communication attempts

Since multiple consecutive unsuccessful OMCI communications with an ONU can also be caused by other circumstances, such as a temporary loss of connection to the OLT involved, it should be avoided to set the ONU to unreachable and to reset the configuration in this case because the core does not ensure that the affected ONUs will be brought back into service after the OLT is available again.