Better error message if A requires B, but A wasn't given

[pacak]

Say your app supports flag --describe that would make it to do whatever it was doing before, but also to describe the computation, it might be possible to have --detailed flag that makes description detailed. It doesn't make sense to specify --detailed without --describe, but naive implementation would result in a less clear error message

struct Describe {
    describe: (),
    detailed: bool,
}

Currently you can make a better error message with guard and Describe containing two bools...