hq server crashes if dashboard is terminated during journal streaming

If a running hq server has a large journal, and the `hq dashboard` command is run it can take many seconds for the dashboard to appear due to streaming of millions of events. It is possible that the hq server crashes if the user Ctrl-C kills the dashboard before journal streaming to client has completed: ```bash thread '<unnamed>' panicked at crates/hyperqueue/src/server/event/journal/stream.rs:99:45: called `Result::unwrap()` on an `Err` value: SendError { .. } ```