memoize_s3 graceful degradation

[atspaeth]

Just a TODO item for myself before publishing some code that relies on it: the @memoize decorator from braingeneers.utils.memoize_s3 should be able to degrade gracefully (and just produce a warning) if the user doesn't have credentials to our S3 buckets.

Might be nice to also have an environment variable flag that asserts the existence of the remote cache, e.g. in k8s jobs, but default behavior should definitely be transparent.