code: private String getArgRequiredErrorMessage() {
          return getMutexTargetList().isEmpty() ?
                  String.format("Argument '%s' is required", getLongName()) :
                  String.format("Argument '%s' is required unless one of {%s} are provided", getLongName(), getMutexTargetList());
      }
docs: Error message for when mutex args are mutually required (meaning one of them must be specified) but none was