message: Add helpful error output for Switch-related toco failure
diff: @@ -92,7 +92,9 @@ bool ResolveTensorFlowSwitch::Run(Model* model, std::size_t op_index) {
  if (*input_it == switch_op->outputs[nonselected_output_index]) {
  // Let us guard our assumption that only Merge nodes consume the outputs
  // of Switch nodes:
  - CHECK(other_op->type == OperatorType::kMerge);
  + CHECK(other_op->type == OperatorType::kMerge)
  + << "Found " << HelpfulOperatorTypeName(*other_op)
  + << " as non-selected output from Switch, but only Merge supported.";
  input_it = other_op->inputs.erase(input_it);
  } else {
  ++input_it;
  