modal: Hide, instead of empty, the dialog error element.

Consider a modal with an error element displayed. When a user clicks
the submit button, the current behaviour is to empty the element. This
empties the inner HTML of the error element, but the CSS makes it still
visible, just empty. This looks bad. To avoid this, we hide the element.
We use the `ui_report` module to display errors, which adds the `show`
class to the error element every time an error is to be displayed. This
makes sure that the error element isn't hidden when the user re-clicks
the submit button while the modal is still open.

static/js/dialog_widget.js

@@ -134,7 +134,7 @@ export function launch(conf) {
         } else if (conf.close_on_submit) {
             close_modal();
         }
-        $("#dialog_error").empty();
+        $("#dialog_error").hide();
         conf.on_click(e);
     });