Livestream exception raising

* Implements an actual check for whether a video is a livestream, and raises an exception if it is.

Author: tfdahlin

pytube/__main__.py

@@ -268,6 +268,8 @@ def check_availability(self):
             elif status == 'ERROR':
                 if reason == 'Video unavailable':
                     raise exceptions.VideoUnavailable(video_id=self.video_id)
+            elif status == 'LIVE_STREAM':
+                raise exceptions.LiveStreamError(video_id=self.video_id)
 
     @property
     def vid_info(self):

pytube/extract.py

@@ -134,6 +134,8 @@ def playability_status(watch_html: str) -> (str, str):
     """
     player_response = initial_player_response(watch_html)
     status_dict = player_response.get('playabilityStatus', {})
+    if 'liveStreamability' in status_dict:
+        return 'LIVE_STREAM', 'Video is a live stream.'
     if 'status' in status_dict:
         if 'reason' in status_dict:
             return status_dict['status'], [status_dict['reason']]