Merge remote branch 'Seldaek/exception_msg'

* Seldaek/exception_msg:
  [Config] Improved exception message a bit

src/Symfony/Component/Config/Definition/ArrayNode.php

@@ -470,7 +470,9 @@ class ArrayNode extends BaseNode implements PrototypeNodeInterface
                 if (!$this->allowNewKeys) {
                     throw new InvalidConfigurationException(sprintf(
                         'You are not allowed to define new elements for path "%s". '
-                       .'Please define all elements for this path in one config file.',
+                       .'Please define all elements for this path in one config file. '
+                       .'If you are trying to overwrite an element, make sure you redefine it '
+                       .'with the same name.',
                         $this->getPath()
                     ));
                 }