do not allow to stop already stopped timesheets (#289)

src/Repository/RepositoryException.php

@@ -0,0 +1,14 @@
+<?php
+
+/*
+ * This file is part of the Kimai time-tracking app.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace App\Repository;
+
+class RepositoryException extends \Exception
+{
+}