Allow sending Error objects to alertError instead of just plain strings.

public/src/app.js

@@ -134,6 +134,8 @@ app.cacheBuster = null;
 	};
 
 	app.alertError = function (message, timeout) {
+		message = message.message || message
+
 		if (message === '[[error:invalid-session]]') {
 			return app.handleInvalidSession();
 		}