I'm not totally sure what the point of this is. This is advertised as exit codes "for humans and machines", however
* humans will always get way more information from a readable error message printed to stderr before exiting
* machines do not care about error details from third-party tools, apart from whether they are an input error (which can be blamed on the user, if they provided said input) or an internal error (including usage error) which is a bug and should be fixed by the calling program's developer
Indeed, the README has a lot of examples of using it in your child program, to report error, but no example of a parent program dealing with the reported error.
* humans will always get way more information from a readable error message printed to stderr before exiting
* machines do not care about error details from third-party tools, apart from whether they are an input error (which can be blamed on the user, if they provided said input) or an internal error (including usage error) which is a bug and should be fixed by the calling program's developer
Indeed, the README has a lot of examples of using it in your child program, to report error, but no example of a parent program dealing with the reported error.