Improved template handling with exceptions

The Page class is now responsible for executing the subpage template
rather than the index file.
Template processing has been improved with exceptions to catch 404s, or
any other template exception, which will abort the current page and
display an error document instead.

pages/errors/404.php

@@ -0,0 +1,9 @@
+<?php
+
+$main = HandBrakeCluster_Main::instance();
+$req  = $main->request();
+
+$this->smarty->assign('requested_page', htmlspecialchars($req->request_string()));
+
+
+?>