Fix the error message for SSO credential providers (#2722)

## Motivation and Context
The current error if the SSO error message is not enabled is inscrutable
and doesn't point the customer towards the removed feature.

## Description
Fix the error message, add tests.

## Testing
tests

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