Add InvalidTransition exception with more helpful message.

[ordinaryzelig]

I thought it would be nice to see and have access to a bit more information when the transition is invalid. This changes the RuntimeError to a custom StatefulEnum::InvalidTransition exception. The error message contains the state and the event that was attempted. Those 2 pieces of information are also stored on the exception itself for easy access.

I almost took it a step further by adding what the state would have been if the event had succeeded – e.g. in the test below: Invalid transition from "resolved" to "assigned" via event :assign!' – but that would take significantly more work. I believe the solution to that would require storing the events in memory. I'm not even sure how useful it would be to know that information. But if you like the idea, I would be happy to continue to explore the idea.

[andrepinho]

This is great! It seems to solves the only task in the TODO section.

I'm currently rescuing from RuntimeError with minimal details about what's going on and this would be very helpful.

I wonder why it has not been merged yet. Does this implementation need some work? Is it just a matter of resolving conflicts? I would love to contribute but I'm not sure how.

[ordinaryzelig]

I would love to contribute but I'm not sure how.

You could start with resolving the conflicts and making sure the tests still pass. 😄

[ordinaryzelig]

Updated to v0.7.0. Resolved conflicts.