Add specific error message for folders with unpermitted name or add a workaround to copy files in it

[v4ssi404]

#31 (comment)

In contrast to iOS, Windows has some limitations on how files and folders can be named. You can read more about it here: https://stackoverflow.com/a/36227372/8868841. Basically, folders are not allowed to be called AUX, or AUx in your case. I can't do much about this. An option would be to export it to a folder with a slightly changed name, but I should definitely add a better error message. I won't have time to do this very soon as I'm busy with studying. But it would be very nice if you could open a new issue for this that I don't forget about it.