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 like "phabricator/ab/cd/efgh" instead of "phabricator/abcdefgh". When I had to go look for the file on S3 it took a few minutes of scrolling since the web interface isn't very fast. Make it so a file can be located by navigating through pieces of the hash.

Test Plan: Viewed an empty file, no fatal. Viewed the file from T2257 locally, no fatal (no data either, but it's gone). Uploaded a file, saw a nice path.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2257

Differential Revision: https://secure.phabricator.com/D4303