Added missing @throws in Twig_LoaderInterface

Based on Twig_Loader_Chain implementation, this seems to be the possible exceptions that can be thrown by loaders.

lib/Twig/LoaderInterface.php

@@ -23,6 +23,8 @@ interface Twig_LoaderInterface
      * @param  string $name The name of the template to load
      *
      * @return string The template source code
+     *
+     * @throws Twig_Error_Loader When $name is not found
      */
     function getSource($name);
 
@@ -32,6 +34,8 @@ interface Twig_LoaderInterface
      * @param  string $name The name of the template to load
      *
      * @return string The cache key
+     *
+     * @throws Twig_Error_Loader When $name is not found
      */
     function getCacheKey($name);
 
@@ -40,6 +44,8 @@ interface Twig_LoaderInterface
      *
      * @param string    $name The template name
      * @param timestamp $time The last modification time of the cached template
+     *
+     * @throws Twig_Error_Loader When $name is not found
      */
     function isFresh($name, $time);
 }