Throw HTTP 403 on extension validation error

The way I read it, HTTP 405 is a generic statement about the
resource. Once a language pack is not the default, this is not
true anymore, so I figured 403 is more correct.

src/Http/Exception/ForbiddenException.php

@@ -0,0 +1,22 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Flarum\Http\Exception;
+
+use Exception;
+
+class ForbiddenException extends Exception
+{
+    public function __construct($message = null, $code = 403, Exception $previous = null)
+    {
+        parent::__construct($message, $code, $previous);
+    }
+}