Return 5XX for requests that land in a reused session that crashes

[ulyssa]

This PR moves the logic for turning an error into a 500 response to happen right after the guest crashes by calling Session::close_downstream_response_sender so that it will still apply to reused sessions. Fortunately, the ways that a guest can exit here are much simpler than in Compute, so this ends up being a simpler change.

[acw]

Seems fine, although I have one question about a panic.

[acw]

Why panic here, rather than turning the execution error into an anyhow response?

[ulyssa]

I'm doing this to continue matching the behaviour from here:

Viceroy/src/execute.rs

Lines 592 to 606 in f2ea186

	Ok(_) => (Response::new(Body::empty()), None),
	Err(ExecutionError::WasmTrap(e)) => {
	event!(
	Level::ERROR,
	"There was an error handling the request {}",
	e.to_string()
	);
	#[allow(unused_mut)]
	let mut response = Response::builder()
	.status(hyper::StatusCode::INTERNAL_SERVER_ERROR)
	.body(Body::empty())
	.unwrap();
	(response, Some(e))
	}
	Err(e) => panic!("failed to run guest: {}", e),