You could keep the old behavior and detect a particular error from PullRequestForBranch and add extra error text:

if err != nil {
  if strings.Contains(err.Error(), "no open pull requests found") {
    err = fmt.Errorf("%s\nTo open a specific pull request, add the pull request number as argument", err)
  }
  return err
}

We can probably use an error like this in situations where no issue or pr is found, like gh pr view 239874987. But I left that for another PR.

Nit: a more Go 1.13-way of matching a specific error type, one that plays well with errors.Wrap https://golang.org/pkg/errors/#As

var notFoundErr *api.NotFoundError
if errors.As(err, &notFoundErr) {
  return fmt.Errorf(...)
}
return err

Nit: you can simplify this implementation via embedding

type NotFoundError struct {
  error
}

// usage:
return NotFoundError{fmt.Errorf(...)}

Then you don't need to define the Error() function.

😮 Nice!