`Queue::close` reports the messages a dead process never read -- they
reached no transcript, so that error is the only place they are ever
mentioned -- but it left each one drawn as a bubble waiting to be read, by
a session that no longer exists. Nothing would ever clear it: the
`UserMessage` that resolves a queued bubble is exactly what is not coming.
Seen on the emulator as a grey bubble sitting under its own error message,
still saying "tap to take it back", on a session reporting `exited`.
It now sends the `MessageDropped` the unqueue route introduced, one per
lost message, alongside the error. The error says what happened and the
drop is what ends it, which is the same division of labour as the rest of
this path.