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

  // caller of event_mgr->ThenExecute() blocks on the completion of the callback
  // (as in the case of ConstOp kernel creation on GPU, which involves copying a
  // CPU tensor to GPU).
  // Setting a larger thread pool does not help with the Swift caller, as we use
  // a different TFE context for each thread of execution (for running graph
  // functions, and their send/recvs corountines).