message: Raise an error when loop execution is cancelled
diff: @@ -602,12 +602,8 @@ LoopCondOp::~LoopCondOp() = default;
  void LoopCondOp::Compute(OpKernelContext* context) {
  CancellationManager* cm = context->cancellation_manager();
  bool already_cancelled = cm->IsCancelled();
  -
  - if (already_cancelled) {
  - Tensor continue_running(false);
  - context->set_output(0, continue_running);
  - return;
  - }
  + OP_REQUIRES(context, !already_cancelled,
  + errors::Cancelled("Loop execution was cancelled."));
  context->set_output(0, context->input(0));
  }
  