docs: public void onFailure(Throwable t) {
  
          if ((t instanceof StatusCodeException) && (((StatusCodeException)t).getStatusCode() == 0)) {
              // a status code 0 indicates the client aborted the request, most likely when leaving the page, this should be ignored
              return;
          } else if ((t instanceof StatusCodeException) && (((StatusCodeException)t).getStatusCode() == 500)) {
              // a server error 500 most likely indicates an expired session and there for insufficient user rights to access any GWT service
              CmsErrorDialog dialog = new CmsErrorDialog(Messages.get().key(Messages.GUI_SESSION_EXPIRED_0), null);
              dialog.center();
          } else {
              CmsErrorDialog.handleException(t);
          }
          // remove the overlay
          stop(false);
      }