When an upload (e.g. a CI artifact) reaches the maximum file size limit,
uploads via S3 would return a 500 error to the user. This made it
difficult to understand why the upload failed.

This was happening because the `hardLimitReader` was aborting the
transfer with `ErrEntityTooLarge`, but this error was wrapped in layers
of AWS errors. Since none of these AWS errors were understood by the
file handler, a 500 error was returned.

To fix this, AWS has a way to retrieve the original error. We now
recursively go down the error stack to find the root cause.

Note that there is an open issue in the AWS SDK to make this easier with
Golang (https://github.com/aws/aws-sdk-go/issues/2820).