fix: Fix downloading a playlist as a TAR archive

This is the only time where youtube-dl returning an empty URL is not a problem.

classes/EmptyUrlException.php

@@ -0,0 +1,15 @@
+<?php
+/**
+ * EmptyUrlException class.
+ */
+
+namespace Alltube;
+
+use Exception;
+
+/**
+ * Exception thrown when youtube-dl returns an empty URL.
+ */
+class EmptyUrlException extends Exception
+{
+}

classes/VideoDownload.php

@@ -155,7 +155,7 @@ class VideoDownload
         $urls = explode("\n", $this->getProp($url, $format, 'get-url', $password));
 
         if (empty($urls[0])) {
-            throw new Exception(_('youtube-dl returned an empty URL.'));
+            throw new EmptyUrlException(_('youtube-dl returned an empty URL.'));
         }
 
         return $urls;