code: private List<Path> checkDirectories(List<Path> pathList) {
          List<Path> result = new ArrayList<>();
          for (Path path : pathList) {
              if (!Files.exists(path)) {
                  LOGGER.debug("'{}' does not exist", path);
              } else if (!Files.isDirectory(path)) {
                  LOGGER.warn("'{}' is not directory", path);
              }
              // todo: should we use not existing paths? this behavior is copied from 'protoc' - it just shows warning
              result.add(path);
          }
          return result;
      }
docs: Check that all elements in the list exist and are directories.
  Log warning for each element that is not directory.