code: public String notOk(String errMsg){
          Map<String, String> notifyParams = new HashMap<>();
          notifyParams.put(WepayField.RETURN_CODE, "FAIL");
          notifyParams.put(WepayField.RETURN_MSG, errMsg);
          return Maps.toXml(notifyParams);
      }
docs: 通知不成功
  @param errMsg 失败消息
  @return 通知失败的XML消息