Added AlreadySubscribedException

Closes #1201

app/Exceptions/AlreadySubscribedException.php

@@ -0,0 +1,24 @@
+<?php
+
+/*
+ * This file is part of Cachet.
+ *
+ * (c) Alt Three Services Limited
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace CachetHQ\Cachet\Exception;
+
+use Exception;
+
+/**
+ * This is the already subscribed exception class.
+ *
+ * @author Graham Campbell <graham@alt-three.com>
+ */
+class AlreadySubscribedException extends Exception implements ExceptionInterface
+{
+    //
+}