[downloader/external] check for ffmpeg availablity when it used for m3u8 download

youtube_dl/downloader/external.py

@@ -175,6 +175,9 @@ def available(cls):
     def _call_downloader(self, tmpfilename, info_dict):
         url = info_dict['url']
         ffpp = FFmpegPostProcessor(downloader=self)
+        if not ffpp.available:
+            self.report_error('m3u8 download detected but ffmpeg or avconv could not be found. Please install one.')
+            return False
         ffpp.check_version()
 
         args = [ffpp.executable, '-y']