Add ServerTransferringException, use is_null

app/Exceptions/Http/Server/ServerTransferringException.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace Pterodactyl\Exceptions\Http\Server;
+
+use Illuminate\Http\Response;
+use Symfony\Component\HttpKernel\Exception\HttpException;
+
+class ServerTransferringException extends HttpException
+{
+    public function __construct()
+    {
+        parent::__construct(Response::HTTP_CONFLICT, 'Server is currently being transferred.');
+    }
+}