Right now, if we are validating a missing piece of a sparse-format entry, we send an If-Range request. That works fine if it succeeds with a 206,
but if it fails with a 200, that's basically just wasting the user's bandwidth, because we can't actually write out a 200 into a 206-entry, 
so we end up either:
1) If we already delivered some bytes, failing the request, and throwing out the 200.
2) If we didn't deliver any bytes, restarting, and throwing out the 200.

Given we don't actually use the body, it would be better to just use If-Match/If-Not-Modified-Since instead and just get a 412.

However, we should still use If-Range for resume of truncated entries, since that's a non-sparse-format entry, so can store a 200 properly.