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

PHPBB-17413

phpBB/phpbb/captcha/plugins/legacy_wrapper.php

@@ -118,6 +118,14 @@ class legacy_wrapper implements plugin_interface
 		return false;
 	}
 
+	/**
+	 * {@inheritDoc}
+	 */
+	public function get_error(): string
+	{
+		return $this->last_error;
+	}
+
 	/**
 	 * {@inheritDoc}
 	 */