Show exit status

[RalfJung]

Programs like this are confusing to run on the playground because they print a panic message, but actually exit successfully:

fn unwind() {
    panic!();
}

fn main() {
    std::panic::catch_unwind(|| unwind()).unwrap_err();
}

It would be good to have some way to see the exit status of the program on playground, to tell whether the program (thinks it) ran successfully or failed.