Timeouts, retries don't work when a state returns True

[cviveiro]

Found this morning, I'd changed a state to return True rather than None, and all of a sudden timeouts in that state didn't work. I changed it back, and they do. @knitHacker looked into it just now and says that when we return True we run the on_leave method, which will reset both the timers and the retries. That seems unexpected to me.

@Jeff-Ciesielski

[Jeff-Ciesielski]

So I think that this should technically be an error. States should return a state, or nothing. I'll update documentation, but would you be comfortable if this just raised a ValueError?

[cviveiro]

SGTM

[knitHacker]

We explicitly had in the comments and functionality that if you return True a message wasn't sent to the trap function.

[knitHacker]

If we didn't return anything at all, or we returned that we swallowed the message, we'll assume that the FSM is no longer busy and is waiting on some external message to move the state along

It's long past time we wrote up some formal specs.