return a meaningful error message on failure.
*   tf.data:
    *   Remove `num_parallel_parser_calls` argument from
        `tf.contrib.data.make_csv_dataset()`. [tf.data] Remove
        `num_parallel_parser_calls` argument from
        `tf.contrib.data.make_csv_dataset()`.
    *   `tf.data.Dataset.list_files()` raises an exception at initialization
        time if the argument matches no files.