[ticket/17413] Make turnstile captcha work on registration page

PHPBB-17413

phpBB/phpbb/captcha/plugins/plugin_interface.php

@@ -73,6 +73,13 @@ interface plugin_interface
 	 */
 	public function validate(): bool;
 
+	/**
+	 * Get error string from captcha
+	 *
+	 * @return string Error string, empty string if there is no error
+	 */
+	public function get_error(): string;
+
 	/**
 	 * Return whether captcha was solved
 	 *