fix: restjson error deserialization when no body is present

[lucix-aws]

Closes #4811.

• allow restjson error deserialization (unmodeled or otherwise) without a response body
• check auxiliary __type field for error code
• update error code sanitization to remove both namespace and details
• much more comprehensive test suite

[isaiahvita]

In the PR description, it states: allow restjson error deserialization (unmodeled or otherwise) without a response body

but it looks like here if we fail to deserialize the error on headers alone, we still fallback to the response body? so how are we allowing error deserialization without a response body?

[lucix-aws]

So the change is to not explode when there's no body. We have and still do fall back to inspecting the body if the headers don't give us the information we need.