code: @Override
      protected void onError(final AjaxRequestTarget _target)
      {
          if (this.errorHandling) {
              final FeedbackCollector collector = new FeedbackCollector(getForm().getPage());
              final List<FeedbackMessage> msgs = collector.collect();
              final StringBuilder html = new StringBuilder()
                              .append("<table class=\"eFapsValidateFieldValuesTable\">");
              for (final FeedbackMessage msg : msgs) {
                  msg.getReporter().add(AttributeModifier.append("class", "invalid"));
                  _target.add(msg.getReporter());
                  Serializable warn = null;
                  if (msg.getMessage() instanceof ValidationErrorFeedback) {
                      // look if a message was set
                      warn = ((ValidationErrorFeedback) msg.getMessage()).getMessage();
                      // still no message, create one
                      if (warn == null) {
                          warn = ((ValidationErrorFeedback) msg.getMessage()).getError().getErrorMessage(
                                          new ErrorMessageResource());
                      }
                  }
                  String label = "";
                  if (msg.getReporter() instanceof IFieldConfig) {
                      label = ((IFieldConfig) msg.getReporter()).getFieldConfig().getLabel();
                  }
                  html.append("<tr><td>").append(label).append(":</td><td>")
                                  .append(warn).append("</td></tr>");
              }
              html.append("</table>");
  
              final ModalWindowContainer modal = ((AbstractContentPage) getComponent().getPage()).getModal();
  
              modal.setInitialWidth(350);
              modal.setInitialHeight(200);
  
              modal.setPageCreator(new ModalWindow.PageCreator()
              {
  
                  private static final long serialVersionUID = 1L;
  
                  @Override
                  public Page createPage()
                  {
                      return new DialogPage(((AbstractContentPage) getComponent().getPage()).getPageReference(),
                                      html.toString(), true, false);
                  }
              });
              modal.show(_target);
          }
      }
docs: Default means nothing is done on error.
  
  @see org.apache.wicket.ajax.form.AjaxFormSubmitBehavior#onError(org.apache.wicket.ajax.AjaxRequestTarget)
  @param _target AjaxRequestTarget