Exit code is always random memory garbage

[rtfeldman]

This is because in host.c we use the return value of rust_main() as the exit code, but rust_main doesn't return anything right now.

To fix this:

• Change rust_main to return i32
• If roc_main's task returns Ok {} then return 0
• If roc_main's task returns an Err then return the 32-bit integer inside it