Add 'as-service-error' to simplify handling of specific error conditions (#3333)

## Motivation and Context
aws-sdk-rust#1010

## Description
Add `as-service-error` helper function to cleanup handling of service
error

## Testing
UT

## 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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