code: private static String buildDescription(final List<String> missingFields) {
      final StringBuilder description =
        new StringBuilder("Message missing required fields: ");
      boolean first = true;
      for (final String field : missingFields) {
        if (first) {
          first = false;
        } else {
          description.append(", ");
        }
        description.append(field);
      }
      return description.toString();
    }
docs: Construct the description string for this exception.