Fixed incorrect return value checks

https://dev.mysql.com/doc/refman/5.7/en/mysql-commit.html https://dev.mysql.com/doc/refman/5.7/en/mysql-rollback.html

Return Values Zero for success. Nonzero if an error occurred.

@saltzmanjoelh Thanks for the pull request, however, would you mind check out the latest version of this repo and do another PR to resolve the conflicts, if possible?

okey dokey, should be good to go @RockfordWei

Sorry FYI, may I also ask a test script for your PR, if possible, something like testTransactions() to prove your valuable work and help the end users to best understand this proposal.

Thank you very much!