Fix crashes in yt_video_metadata

- yt_video_metadata returned a generic non-json-api-friendly 500 error
  when called on a non-youtube video
- load_metadata_from_youtube was crashing when called from the xblock
  yt_video_metadata endpoint. It passes a webob request, which has a
  different api for retrieving the http referer.

common/lib/xmodule/xmodule/video_module/video_handlers.py

@@ -391,6 +391,10 @@ class VideoStudentViewHandlers(object):
         runtime uses a similar REST API that's not an XBlock handler.
         """
         from lms.djangoapps.courseware.views.views import load_metadata_from_youtube
+        if not self.youtube_id_1_0:
+            # TODO: more informational response to explain that yt_video_metadata not supported for non-youtube videos.
+            return Response('{}', status=400)
+
         metadata, status_code = load_metadata_from_youtube(video_id=self.youtube_id_1_0, request=request)
         response = Response(json.dumps(metadata), status=status_code)
         response.content_type = 'application/json'

lms/djangoapps/courseware/views/views.py

@@ -308,7 +308,16 @@ def load_metadata_from_youtube(video_id, request):
         yt_timeout = settings.YOUTUBE.get('TEST_TIMEOUT', 1500) / 1000  # converting milli seconds to seconds
 
         headers = {}
-        http_referer = request.META.get('HTTP_REFERER')
+        http_referer = None
+
+        try:
+            # This raises an attribute error if called from the xblock yt_video_metadata handler, which passes
+            # a webob request instead of a django request.
+            http_referer = request.META.get('HTTP_REFERER')
+        except AttributeError:
+            # So here, let's assume it's a webob request and access the referer the webob way.
+            http_referer = request.referer
+
         if http_referer:
             headers['Referer'] = http_referer