Add themed error pages

- Module init, post and rawContent-triggered HTTPException generate the classic bare HTTP status page
- Module content-triggered HTTPException generate themed error pages
- Trim System::httpExit to the bare minimum

src/Module/PageNotFound.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace Friendica\Module;
+
+use Friendica\BaseModule;
+use Friendica\Core\L10n;
+use Friendica\Network\HTTPException;
+
+class PageNotFound extends BaseModule
+{
+	public static function content()
+	{
+		throw new HTTPException\NotFoundException(L10n::t('Page not found.'));
+	}
+}