message: fix metal build
  there is not message(). use error_message() instead
diff: @@ -578,7 +578,7 @@ TfLiteStatus DelegatePrepare(TfLiteContext* context, TfLiteDelegate* delegate) {
  // forbids that.
  const auto status = metal_delegate->Prepare(context, params);
  if (status.ok()) return metal_delegate;
  - context->ReportError(context, "TfLiteGpuDelegate Prepare: %s", status.message().data());
  + context->ReportError(context, "TfLiteGpuDelegate Prepare: %s", status.error_message().c_str());
  return nullptr;
  },
  // .free
  @@ -591,7 +591,7 @@ TfLiteStatus DelegatePrepare(TfLiteContext* context, TfLiteDelegate* delegate) {
  [](TfLiteContext* context, TfLiteNode* node) -> TfLiteStatus {
  const auto status = GetMetalDelegate(node)->Invoke(context);
  if (status.ok()) return kTfLiteOk;
  - context->ReportError(context, "TfLiteMetalDelegate Invoke: %s", status.message().data());
  + context->ReportError(context, "TfLiteMetalDelegate Invoke: %s", status.error_message().c_str());
  return kTfLiteError;
  },
  nullptr, // .profiling_string
  