Global defaultResponse/error

### Clear and concise description of the problem

It would be great if it was possible to globally define an Error type that all operations (or maybe just all operations within a namespace) use as a default response for OpenAPI.

This would enable defining something like a `ServerError` type that acts as a fallback, instead of having to use `Thing | NotFoundResponse | ServerError` for every operation.

I'm not sure what this would look like code-wise, but I can try to come up with some imaginary code examples if you'd like to explain the feature request further.

### Checklist

- [X] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [X] Read the [docs](https://typespec.io/docs/).
- [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.