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.

templates/errors/404.tpl

@@ -0,0 +1,6 @@
+<h2>The requested page could not be found</h2>
+<p>
+	The file you requested ({$requested_page}) could not be found.
+	If you typed in the address manually, check that you have spelled it correctly,
+	or if you followed a link, let us know and we'll look into it. 
+</p>