Bad exit code

[bhansconnect]

Currently, we aren't correctly setting the exit code and it can be random.

In lib.rs, we have pub extern "C" fn rust_main(). It does not return an exit code.
Then in host.c we have int main() { return (int)rust_main(); }. We take the void return value and cast it to an int.

we should either change that to:

int main() {
    rust_main();
    return 0;
}

Or we should change rust_main to return an int.