Allow <Error> to trigger error handling for S3 (#2958)

## Motivation and Context
- https://github.com/awslabs/aws-sdk-rust/issues/873

## Description
Add a customization for S3 so that if `<Error>` is the root element, we
trigger the error parsing flow

## Testing
- Added an integration test

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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