code: public static ServiceUnavailable of(int errorCode) {
          if (_localizedErrorMsg()) {
              return of(errorCode, defaultMessage(SERVICE_UNAVAILABLE));
          } else {
              touchPayload().errorCode(errorCode);
              return _INSTANCE;
          }
      }
docs: Returns a static ServiceUnavailable instance and set the {@link #payload} thread local
  with error code and default message.
  
  When calling the instance on {@link #getMessage()} method, it will return whatever
  stored in the {@link #payload} thread local
  
  @param errorCode the app defined error code
  @return a static ServiceUnavailable instance as described above