Set exit code when commands fail

[mmarchini]

If I run git node land on a new machine with nothing configured, I'll get the instructions to set required variables, and then the process will exit with status code 0. In order for us to be able to use ncu on a Commit Queue, it needs to return the appropriate status codes when something goes wrong, otherwise automation won't work.

(I didn't check which other failures will exit with 0, but all failures should set the exit code to a non-zero code)

[codebytere]

Closed by #443

[mmarchini]

I'd like to keep this open because there are still many situations where the exit code will return zero even if the command failed. Would be good for us to list those as a next step.

[github-actions]

This issue is stale because it has been open many days with no activity. It will be closed soon unless the stale label is removed or a comment is made.