Support failure handling with Axum

[ChetanBhasin]

Currently if your axum handler returns a Json<T>, that's perfectly fine, but Axum also supports returning Result<Json<T>>, in which case we fail.
I was wondering if there is a way to support that.

I'd be happy to help out.

[kurtbuilds]

No support currently, but definitely a welcome feature.

I think the main difficulty is knowing what errors correspond to what error codes.

If you have a proposal, or you want to share more about your use case, happy to workshop it with you and accept any PRs for it.