add retries to S3 GetChunk and PutChunk

[Joe-Improbable]

This PR should solve (or at least significantly reduce the chance of) #175

It changes the S3 store to also respect the error-retry option and retry any failed GET or PUT requests

[folbricht]

Thank you. I'm wondering if longer-term it'd be better to use a wrapper that implements the Store interface to do the retries, rather than having it in every store.