return first exited process exit code

[corpix]

This patch enhances existing "fail-fast" behavior. While hivemind already exits when the first process terminates, this change ensures hivemind own exit code mirrors that of the first process to finish.

My main motivation is: I use hivemind in CI workflows to start multiple parallel processes, it is crucial to know if a group of processes finished successfully or with error. I am aware of this PR https://github.com/DarthSim/hivemind/pull/39, but it has different semantics, suggesting highest exit code instead of exit code of first finished process.