Revise unhandled error variant according to RFC-39 (#3191)

This PR implements
[RFC-39](https://github.com/smithy-lang/smithy-rs/blob/main/design/src/rfcs/rfc0039_forward_compatible_errors.md)
with a couple slight deviations:
- No `introspect` method is added since `Error` already implements
`ProvideErrorMetadata`.
- The same opaqueness and deprecation pointer is applied to the enum
unknown variant for consistency.

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