If there are no valid data to save, exit with an error.
E.g., telling it to save an unused (type 0) partition. Previously, we would create a single valid range with start at the beginning of disk (or partition) and a size of zero. However, size zero is taken by the imagezip compressor to mean "til EOF" so we would wind up creating a really large image instead of doing nothing. It is debatable whether this is really an error. Perhaps we should just create a valid image file with one chunk containing no valid ranges, but it seems likely that they probably made a mistake and we should call it out immediately.