ICS 04: timeoutChannelUpgrade should write error receipt

[colin-axner]

The timeoutChannelUpgrade should write an error receipt upon successfully timing out an upgrade. While not strictly necessary, it potentially reduces handshake steps by 1. In the case of crossing INIT's, 1 side might timeout before the other. If this occurs, without the error receipt, the counterparty would either need to force a error receipt by calling the TRY step or wait for it's own timeout to exceed its deadline.

By writing the error receipt upon timeout, you don't require a TRY to be executed just to obtain an error receipt to cancel the upgrade

In the non-crossing INIT case, such as a single chain calling INIT and then timeout, an error receipt isn't useful, but it should not cause any issues either. In fact, it could simply allow a chain to synchronize upgrade sequences:
chain A, sequence 1, INIT, moves to sequence 2
chain A, timeout, error receipt for sequence 2 written

chain B, INIT, moves to sequence 2
chain B, cancel on error receipt, write error receipt

(without the error receipt written on timeout, chain A would have needed to call TRY to force an error receipt for sequence 2, note chain A could not respond to sequence 2 since it already attempted that sequence for a different upgrade)

chain B, INIT, moves to sequence 3
chain A, TRY, success