code: private String getErrorMessage(Throwable throwable) {
  
          StringBuffer result = new StringBuffer(512);
  
          result.append(
              CmsVaadinUtils.getMessageText(org.opencms.ui.components.Messages.GUI_ERROR_DIALOG_MESSAGE_0)).append(
                  "<br />");
  
          // if a localized message is already set as a parameter, append it.
          result.append(getMessage(throwable));
  
          return result.toString();
      }
docs: Returns the error message to be displayed.<p>
  
  @param throwable the throwable
  
  @return the error message to be displayed