Added an S3 retry loop to comeback itself.

We're getting a new class of errors; it's becoming annoying to special
case them in the S3 repo, which is maybe not tbe best idea anyway:

    Storing chunk 0: Set: StoreObject: SendRequest: http.DefaultClient.Do: Put https://s3-ap-southeast-2.amazonaws.com/com.github.jacobsa.comeback.jacobsa/blob:ad2b5d7c5e81a2dda2a6adc443a1004e04d02d39: write tcp 54.240.195.151:443: broken pipe