On Windows the write watch is dropped in whole 64 KiB blocks
(WriteWatchCoverage::Exclude), but the unwatch hook was given the page
range of the import. An image with results pending in the same block as
an imported page, outside that page, lost the watch without its guest
bytes saved, and storing its results threw.
The hook now gets the span of blocks Exclude drops (ExcludedSpan, the same
block math), so every image with results pending in those blocks saves its
bytes first.
Dean Galvin