Fixes #4887

To test this using localstack would require adding IAM to our container setup, then creating and setting up users with the desired permissions via the SDK. With all that setup, it might be inconsistent with the real S3 anyway. I thought it best to test manually instead by:

1. Creating user in sandbox with S3 read permissions
2. Pointing integration tests to sandbox S3 with access keys from my admin user
3. Pointing the dockerised delta to sandbox S3 with access keys from the restricted user
4. Run S3StorageSpec, see what file upload failures look like

In the end we now return 403 with

   {
     "@context" : "https://bluebrain.github.io/nexus/contexts/error.json",
     "@type" : "BucketAccessDenied",
     "reason" : "File 'http://delta:8080/v1/resources/6k0mab3nc23xhan/ykab7aepbgphlt0/_/attachment.json' could not be saved using storage 'https://bluebrain.github.io/nexus/vocabulary/mys3storage'",
     "details" : "Access denied to bucket da7goncyy5irq8u at key myprefix/6k0mab3nc23xhan/ykab7aepbgphlt0/files/6/1/6/a/d/7/0/a/attachment.json"
   }