On Windows, when tmpdir is not writable, there are only messages
like this:

2017-07-05 14:04:25 3860 [ERROR] InnoDB: Unable to create temporary file; errno: 0

On other platforms, there would be two messages for each failure:

2017-07-05 17:23:02 140436573771648 [ERROR] mysqld: Can't create/write to file '/dev/null/nonexistent/ibaajU4U' (Errcode: 20 "Not a directory")
2017-07-05 17:23:02 140436573771648 [ERROR] InnoDB: Unable to create temporary file; errno: 20