Extract debug info for traceback

[bbangert]

Per @alexcrichton's comment, the state machine ShutdownCleanup should print the fully extracted error like the code does here:

autopush/autopush_rs/src/server.rs

Lines 310 to 317 in 5bf2de6

	if let Err(e) = res {
	let mut error = e.to_string();
	for err in e.iter().skip(1) {
	error.push_str("\n");
	error.push_str(&err.to_string());
	}
	error!("{}: {}", addr, error);
	}