R1:4f933d34f5ea

Improve S3 integration

Summary:
- Fixes T2257. We wrote a 0-length file (erroneously?) and currently throw when retrieving it. This also happens if you intentionally upload an empty file. I'm not sure what happened with the image: we check for errors during the write, so its existence implies S3 told us the write was successful and then lost the data. Since this is a one-off, I'm not too worried about it. The indistinguishable case of an actually empty file is fixed, at least.
- Writes to a directory…