Work in Progress

hmm.. It's actually still a number even the exit code is 0.

So the the callback is called here: https://github.com/virgo-agent-toolkit/luvit-pkg/pull/1/files#diff-2bbcf2a15d7bbe6942e9b79dc32c31d6R77 , which is relayed here: https://github.com/virgo-agent-toolkit/luvit-pkg/pull/1/files#diff-62e71b4893192129a24189e02ab6168dR29 , i.e., it's the `exit` event fired by `uv.Process`. Maybe we could change `err` to `exit_status` to make it clearer?