signal errors to w.when_code() waiters too
signal errors to w.when_code() waiters too

This fixes the case where "wormhole send" would wait forever (upon network
error) instead of terminating with a useful error message.

Testing this will have to wait until we land the branch that abandons the
wormhole if the first connection fails, since that's the easiest way to
provoke a network error before when_code() has fired.

warner/magic-wormhole,warner/magic-wormhole,warner/magic-wormhole,warner/magic-wormhole