Add missing file

core/Exception/NotYetInstalledException.php

+<?php
+/**
+ * Piwik - free/libre analytics platform
+ *
+ * @link http://piwik.org
+ * @license http://www.gnu.org/licenses/gpl-3.0.html GPL v3 or later
+ *
+ */
+namespace Piwik\Exception;
+
+class NotYetInstalledException extends Exception
+{
+}