S3BinaryCacheStore: Don't use the transfer status callback

This callback is executed on a different thread, so exceptions thrown
from the callback are not caught:

  Aug 08 16:25:48 chef hydra-queue-runner[11967]: terminate called after throwing an instance of 'nix::Error'
  Aug 08 16:25:48 chef hydra-queue-runner[11967]:   what():  AWS error: failed to upload 's3://nix-cache/19dbddlfb0vp68g68y19p9fswrgl0bg7.ls'

Therefore, just check the transfer status after it completes. Also
include the S3 error message in the exception.