This change was to make the code mode in line with [the spec](https://github.com/Coinffeine/coinffeine/wiki/Exchange-algorithm). An exchange has N steps in which the bitcoins are being transferred, and then a final step (called `N+1` in the spec) which returns the deposits to the parties.

This message represents entering into a state to handle this last offer, which is not a step and is calculated differently for the step offers.