Fix #136:  roll back transactions on exceptions.

Cherry-pick:  3221122ad1eff0e96636b440d996226c737890e8
Fix typo, s/derault/default.

Feedback from @silvolu.
Clarify sentence.

Feedback from @silvolu.
Merge pull request #202 from tseaver/136-rollback_on_exception-redux

Fix #136: roll back transactions on exception (redux)