message: Show friendlier error message on failure in tf_optimizer.i
  Without it we trigger a segmentation fault, but later in a different stack, which is not so helpful.
diff: @@ -103,6 +103,11 @@ PyObject* TF_OptimizeGraph(
  std::unique_ptr<tensorflow::grappler::GrapplerItem> grappler_item =
  tensorflow::grappler::GrapplerItemFromMetaGraphDef(graph_id, metagraph, item_config);
  + if (!grappler_item) {
  + TF_SetStatus(out_status, TF_INVALID_ARGUMENT, "Failed to import metagraph, check error log for more info.");
  + return nullptr;
  + }
  +
  tensorflow::DeviceBase* cpu_device = nullptr;
  tensorflow::GraphDef out_graph;
  tensorflow::grappler::MetaOptimizer optimizer(cpu_device, rewriter_config);
  