private String getRequiredArg(String key, String description) {
    String value = getArg(key);
    if (value == null) {
      throw new IllegalArgumentException("Missing required parameter: " + key + " (" + description + ")");
    }
    return value;
  }
Returns a {@link Path} parsed from {@code key} argument which may or may not exist.
planetiler-core/src/main/java/com/onthegomap/planetiler/config/Arguments.java