Summary:
Based on feedback from spetrunia, we should better differentiate error statuses for transaction failures.

https://github.com/MySQLOnRocksDB/mysql-5.6/issues/86#issuecomment-124605954

Test Plan: unit tests

Reviewers: rven, kradhakrishnan, spetrunia, yhchiang, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D43323