Add the possibility to customize error messages

[svanhesteren]

The error messages that come from for ex: ensure_state_may_transition_to! are always like
state may not transition from accepted to cancelled
But sometimes you want a nicer error message and/or specific error messages per individual state change.

Just like you can customize error messages on validations in rails it would be nice to have that here as well.

[julik]

This should be solved in d568cbc - now there is an InvalidTransition exception. It has attributes a custom exception message can be constructed from. We do surface invalid transitions as validation errors though, this will need some thinking - because the message depends on which state you are coming from and which state you were going to