code: protected void onError(Throwable throwable)
      {
          throwable.printStackTrace();
          JOptionPane.showMessageDialog(null,
                                        throwable.getMessage(),
                                        throwable.getClass().getName(),
                                        JOptionPane.ERROR_MESSAGE);
      }
docs: This method is invoked, on the Event Dispatch Thread, if there is an
  exception or error executing the {@link #performTask()} method.
  This default implementation displays a message dialog with details of the
  exception.  It may be over-ridden in sub-classes.
  @param throwable The exception or error that was thrown while executing
  the task.