Don't explode if a forum content piece is by a user that doesn't exist

lms/djangoapps/django_comment_client/utils.py

@@ -301,8 +301,12 @@ def permalink(content):
 def extend_content(content):
     roles = {}
     if content.get('user_id'):
-        user = User.objects.get(pk=content['user_id'])
-        roles = dict(('name', role.name.lower()) for role in user.roles.filter(course_id=content['course_id']))
+        try:
+            user = User.objects.get(pk=content['user_id'])
+            roles = dict(('name', role.name.lower()) for role in user.roles.filter(course_id=content['course_id']))
+        except user.DoesNotExist:
+            logging.error('User ID {0} in comment content {1} but not in our DB.'.format(content.get('user_id'), content.get('id')))
+        
     content_info = {
         'displayed_title': content.get('highlighted_title') or content.get('title', ''),
         'displayed_body': content.get('highlighted_body') or content.get('body', ''),