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/unhandled-exception.tpl

@@ -0,0 +1,36 @@
+<h2>An unhandled error has occurred</h2>
+<p>
+	There was a problem trying to complete the requested action. Please try again and if the problem persists, let us know.
+</p>
+
+{if $display_exceptions}
+<p>
+	An unhandled exception was caught during the page template processing. The full details are shown below:
+</p>
+<table class="exception-details">
+	<colgroup id="header">
+		<col />
+	</colgroup>
+	<colgroup>
+		<col />
+	</colgroup>
+	<tbody>
+		<tr>
+			<th>Exception</th>
+			<td>{$exception_type}</td>
+		</tr>
+		<tr>
+			<th>File</th>
+			<td>{$exception->getFile()}</td>
+		</tr>
+		<tr>
+			<th>Line</th>
+			<td>{$exception->getLine()}</td>
+		</tr>
+		<tr>
+			<th>Message</th>
+			<td>{$exception->getMessage()}</td>
+		</tr>
+	</tbody>
+</table>
+{/if}