public DocumentProtocolRoutingPolicy createPolicy(String param) {
            ExternPolicy ret = new ExternPolicy(param);
            String error = ret.getError();
            if (error != null) {
                return new ErrorPolicy(error);
            }
            return ret;
        }
@author Simon Thoresen Hult
@author jonmv
documentapi/src/main/java/com/yahoo/documentapi/messagebus/protocol/RoutingPolicyFactories.java