[Routing] improved exception message when giving an invalid route name.

src/Symfony/Component/Routing/RouteCollection.php

@@ -95,7 +95,7 @@ class RouteCollection implements \IteratorAggregate
     public function add($name, Route $route)
     {
         if (!preg_match('/^[a-z0-9A-Z_.]+$/', $name)) {
-            throw new \InvalidArgumentException(sprintf('Name "%s" contains non valid characters for a route name.', $name));
+            throw new \InvalidArgumentException(sprintf('The provided route name "%s" contains non valid characters. A route name must only contain digits (0-9), letters (A-Z), underscores (_) and dots (.).', $name));
         }
 
         $parent = $this;