Ensure empty video string deletes the video record and doesn't create

a partial descriptor. 
Use 400 not the contrived 515 for malformed html.

cms/djangoapps/contentstore/views.py

@@ -986,7 +986,7 @@ def course_info_updates(request, org, course, provided_id=None):
         try:
             return HttpResponse(json.dumps(update_course_updates(location, request.POST, provided_id)), mimetype="application/json")
         except etree.XMLSyntaxError:
-            return HttpResponse("Failed to save: malformed html", status=515, content_type="text/plain")
+            return HttpResponseBadRequest("Failed to save: malformed html", content_type="text/plain")
 
 
 @expect_json

cms/djangoapps/models/settings/course_details.py

@@ -163,8 +163,10 @@ class CourseDetails:
     def recompose_video_tag(video_key):
         # TODO should this use a mako template? Of course, my hope is that this is a short-term workaround for the db not storing
         # the right thing
-        result = '<iframe width="560" height="315" src="http://www.youtube.com/embed/' + \
-            video_key + '?autoplay=1&rel=0" frameborder="0" allowfullscreen=""></iframe>'
+        result = None
+        if video_key:
+            result = '<iframe width="560" height="315" src="http://www.youtube.com/embed/' + \
+                video_key + '?autoplay=1&rel=0" frameborder="0" allowfullscreen=""></iframe>'
         return result