message: Add error handling of mlir::openOutputFile
  Dump error message on creation failure.
diff: @@ -149,7 +149,12 @@ int main(int argc, char **argv) {
  lower_tensor_list_ops, &result, &pm);
  if (!status.ok()) return kTrFailure;
  - auto output = mlir::openOutputFile(output_file_name);
  + std::string error_msg;
  + auto output = mlir::openOutputFile(output_file_name, &error_msg);
  + if (output == nullptr) {
  + llvm::errs() << error_msg << '\n';
  + return kTrFailure;
  + }
  output->os() << result;
  output->keep();
  