Handle PermissionError in get_open_files()

get_open_files() is used by the AutoLocker to find files currently opended on the OS. We use it to lock/unlock documents on the server while DirectEditing.

But if the user does not have enough access rights to some file, the whole function will throw such error:

PermissionError: [WinError 5] Access is denied: 'xxx'

The fix is quite simple: skip those files.