Add a `private` field to `WP_Plugin_Install_List_Table`, `$error`.

See #30891.


git-svn-id: https://develop.svn.wordpress.org/trunk@31164 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/class-wp-plugin-install-list-table.php

@@ -13,6 +13,8 @@ class WP_Plugin_Install_List_Table extends WP_List_Table {
 	public $orderby = null;
 	public $groups = array();
 
+	private $error;
+
 	public function ajax_user_can() {
 		return current_user_can('install_plugins');
 	}