code: private String writeGenericFailureInfo(final Exec.Result result)
        throws IOException {
      StringBuilder sb = new StringBuilder();
      sb.append("graphviz process returned: ").append(result.getStatus()).append("\n").append("stdout:\n");
      Files.lines(result.getStdout()).forEach(sb::append);
      sb.append("stderr:\n");
      Files.lines(result.getStderr()).forEach(sb::append);
      return sb.toString();
    }
docs: Writes error info to a String.