>The first is that our error message may not be correct. It’s possible that the exception we’ve caught was not introduced by opening this file, and, the errno may not reflect the errno at the time fopen was called.

All that is needed is a better File_error type that includes the error that happened.

  void f(string s)
  {
      try {
          File_handle fh { s, "r"};
          // use fh
      } catch (const File_error& e) {
          fprintf(stderr, "File error: %s\n", e.msg.c_str();
          return;
      }
  }