What should happen if deliver_cancel raises an exception? In the current implementation, the shielded cancel scope will prevent it from propagating until the process exits, which might take a while if the crash occurred before signaling the process in any way. Maybe on exception from a user-specified deliver_cancel we should call the default deliver_cancel to kill the process? Or just kill() since

Steps to reproduce

• futures-preview = { version = "0.3.0-alpha.19", features = [ "async-await" ] } in Cargo.toml as recommended in §3.1.
• Try to build the example in https://book.async.rs/tutorial/sending_messages.html

use futures::channel::mpsc; // 1
use futures::sink::SinkExt;
use std::sync::Arc;

type Sender<T> = mpsc::UnboundedSender<T>; // 2
type Receiver<T> =