Transition shortcut breaking changes

Addresses concerns raised in #13.

**This is breaking change to the API!**

@brianvh @charlierudolph please review when you get a chance.
Hmm... I can see how this might come across as confusing. The system was entirely transition based, so all errors used to be based off of how the specified transition reacted. When support for events was added, we kind of grew in a way that lives somewhat parallel to transitions, which is were that extra error condition comes from.

I agree that this confusion needs to be addressed, but it may be the best time for that is after this change. Perhaps, when the extraction of events takes place is the right time to address the errors that are thrown. The more I think about it, the more I feel like nearly all errors need to be expressed as coming from the event, rather than the transition...