transcribe._sleep: make it usable from deallocate()
transcribe._sleep: make it usable from deallocate()

If we're closing because of an error, we need to sleep through the old
error, to be able to wait for the "deallocated" message. This might want
to be different: maybe clear the error first, or store the errors in a
list and sleep until a second error happens.

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