fixed id case when setting a service (also slightly enhanced an error message)

Author: fabpot

ContainerBuilder.php

@@ -347,15 +347,15 @@ public function getScopeChildren()
      */
     public function set($id, $service, $scope = self::SCOPE_CONTAINER)
     {
+        $id = strtolower($id);
+
         if ($this->isFrozen()) {
             // setting a synthetic service on a frozen container is alright
             if (!isset($this->definitions[$id]) || !$this->definitions[$id]->isSynthetic()) {
-                throw new BadMethodCallException('Setting service on a frozen container is not allowed');
+                throw new BadMethodCallException(sprintf('Setting service "%s" on a frozen container is not allowed.', $id));
             }
         }
 
-        $id = strtolower($id);
-
         unset($this->definitions[$id], $this->aliases[$id]);
 
         parent::set($id, $service, $scope);