Ensure account creation succeeds before creating comments service user

Also improves handling in cases where account creation did succeed, but
comments service user creation did not.

Depends on commit 88abdd8a0b20bc9816f487b25abdea1953f5661d in https://github.com/edx/cs_comments_service

JIRA: FOR-522

lms/lib/comment_client/user.py

@@ -80,7 +80,16 @@ class User(models.Model):
         retrieve_params = self.default_retrieve_params
         if self.attributes.get('course_id'):
             retrieve_params['course_id'] = self.course_id
-        response = perform_request('get', url, retrieve_params)
+        try:
+            response = perform_request('get', url, retrieve_params)
+        except CommentClientRequestError as e:
+            if e.status_code == 404:
+                # attempt to gracefully recover from a previous failure
+                # to sync this user to the comments service.
+                self.save()
+                response = perform_request('get', url, retrieve_params)
+            else:
+                raise
         self.update_attributes(**response)