Add newsletter management functionality

templates/Error.php

@@ -0,0 +1,22 @@
+<?
+
+if(!$exception){
+	return;
+}
+
+$exceptions = [];
+
+if($exception instanceof Exceptions\ValidationException){
+	$exceptions = $exception->Exceptions;
+}
+else{
+	$exceptions[] = $exception;
+}
+?>
+<ul class="error">
+	<? foreach($exceptions as $ex){ ?>
+	<li>
+		<p><? $message = $ex->getMessage(); if($message == ''){ $message = 'An error occurred.'; } ?><?= Formatter::ToPlainText($message) ?></p>
+	</li>
+	<? } ?>
+</ul>