message: [tf.data] Improve input pipeline cloning error message.
diff: @@ -105,8 +105,14 @@ void DatasetToGraphOp::Compute(OpKernelContext* ctx) {
  (external_state_policy_ == ExternalStatePolicy::kFail);
  GraphDef graph_def;
  - OP_REQUIRES_OK(
  - ctx, AsGraphDef(ctx, dataset, SerializationContext(params), &graph_def));
  + Status s = AsGraphDef(ctx, dataset, SerializationContext(params), &graph_def);
  + if (!s.ok()) {
  + ctx->CtxFailure(errors::FailedPrecondition(
  + "Failed to clone the input pipeline because the input pipeline graph "
  + "could not be serialized: ",
  + s.error_message()));
  + return;
  + }
  // In case we allow stateful ops, we walk the graph and find all the stateful
  // ops in the Graph. We then log a warning indicating what ops' state we are
  // going to throw away.
  