code: private void sanityCheck(List<Path> resources) {
      Set<String> fileNames = resources.stream()
          .map(p -> p.getFileName().toString())
          .collect(Collectors.toSet());
      for (String name : FILENAMES_MUST_EXIST) {
        if (!fileNames.contains(name)) {
          String msg = String.format("CLDR DATA MISSING! Can't find '%s'. "
              + " You may need to run 'git submodule update'", name);
          throw new RuntimeException(msg);
        }
      }
    }
docs: Blow up if required paths do not exist.