User requested rollbacks

I have a use case where I want to a dry run of the transaction, and rollback right before returning. The docs are not very clear about the preferred way to achieve this. I decided to return my own error object that I can check for to know that it isn't a real error.

It would be nicer if this library defined a special `ErrRollback` value, and if it sees that value, rollback the transaction instead of committing, and then also don't return any error to the outer function. It would simplify the error handling code a bit.