objects: make `URL()` respect `s3-insecure`

Helpful in development environments where a local S3 instance is used
without HTTPS.

Don't uppercase error messages

objects: bring back support for plain HTTP

This got lost during the transition from minio to S3 client.

objects: improve support for our s3 deployment

Not sure how well this generalizes but we can improve it when someone
complains.

Switch from minio to AWS SDK S3 client

Moves s3 => objects to avoid conflicting with the AWS module.