fix: Throw an exception when trying to download DASH with ffmpeg

Fallback to default format when we can't download bestaudio

Fixes #165

tests/VideoDownloadTest.php

@@ -406,6 +406,17 @@ class VideoDownloadTest extends TestCase
         $this->download->getAudioStream($url, $format);
     }
 
+    /**
+     * Test getAudioStream function with a DASH URL.
+     *
+     * @return void
+     * @expectedException Exception
+     */
+    public function testGetAudioStreamDashError()
+    {
+        $this->download->getAudioStream('https://vimeo.com/251997032', 'bestaudio/best');
+    }
+
     /**
      * Assert that a stream is valid.
      *