Improve error message.

One of the cases was not explained in the existing error message.

Fixes #3773

lib/Cake/Routing/Router.php

@@ -194,7 +194,7 @@ protected static function _validateRouteClass($routeClass) {
 			$routeClass !== 'CakeRoute' &&
 			(!class_exists($routeClass) || !is_subclass_of($routeClass, 'CakeRoute'))
 		) {
-			throw new RouterException(__d('cake_dev', 'Route classes must extend CakeRoute'));
+			throw new RouterException(__d('cake_dev', 'Route class not found, or route class is not a subclass of CakeRoute'));
 		}
 		return $routeClass;
 	}