lib: Add HttpNotFoundException

The HttpNotFoundException should be used for sending a HTTP 404 response w/ a custom message.

There's also Zend_Controller_Action_Exception but this exception will always show 'Page not found' because we
want to hide messages generated by Zend, like 'Action "foobar" does not exist and was not trapped in __call()'.

refs #6281

library/Icinga/Exception/HttpNotFoundException.php

@@ -0,0 +1,11 @@
+<?php
+
+namespace Icinga\Exception;
+
+/**
+ * Exception thrown for sending a HTTP 404 response w/ a custom message
+ */
+class HttpNotFoundException extends IcingaException
+{
+
+}