Return error if blob download is incomplete by validating hash digest.

[kchilleri]

Closes #1758

[reidpr]

Great work; you've solved the essence of the issue.

See in-line for some other comments.

Also — how might we test this? It's a fairly rare case for the error to come up naturally, so the new code path won't be exercised much unless we add test(s). I have some ideas that we can talk about, but first I'd like you to noodle on it some.

[reidpr]

Great work. The logic seems quite solid to me. I have some style recommendations in-line.

One oversight, unless I’ve missed something: request() only checks the digest if the response is going to a file, i.e. out != None. I think this is fine, but the arguments need to be checked. As written, if out == None and expdigest != None, the latter will be silently ignored. It should instead fail noisily. assert would be fine I think.

[reidpr]

Nice. I didn’t see an assert so I added one.