Comment 5 for bug 557388

Apparently e.g. unetbootin fails to copy this file, which is another source of this kind of problem. See bug 837186. While I'm going to stop ubiquity crashing in this case, (a) it would be good for performance if unetbootin copied this file, and (b) I can't retroactively fix old Ubuntu images but modern unetbootin code might well be used with older Ubuntu images.