Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/contact/src/MailHandlerException.php

@@ -0,0 +1,13 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\contact\MailHandlerException.
+ */
+
+namespace Drupal\contact;
+
+/**
+ * Exception thrown by MailHandler when unable to determine message recipient.
+ */
+class MailHandlerException extends \RuntimeException {}