Add support for locking backups to prevent any accidental deletions

app/Exceptions/Service/Backup/BackupLockedException.php

@@ -0,0 +1,16 @@
+<?php
+
+namespace Pterodactyl\Exceptions\Service\Backup;
+
+use Pterodactyl\Exceptions\DisplayException;
+
+class BackupLockedException extends DisplayException
+{
+    /**
+     * TooManyBackupsException constructor.
+     */
+    public function __construct()
+    {
+        parent::__construct('Cannot delete a backup that is marked as locked.');
+    }
+}