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
Merge pull request #2957 from edx/jsa/forum-user-sync

Ensure account creation succeeds before creating comments service user