Enable mapping h2 errors to correct Status codes.

[LucioFranco]

We currently have this code https://github.com/hyperium/tonic/blob/master/tonic/src/status.rs#L344 which maps h2::Error into proper Code's. We should test and ensure that if the user enables the transport feature we can downcast the errors into h2 variants and mapping them to correct status codes.

[SkamDart]

Happy to take this over but after reading through the code, it looks as if maybe this has already been done with this feature and test?

Unless I'm misunderstanding this and you were looking to feature gate something like, pub enum Http2Error based on this spec under transport?

[LucioFranco]

Last I remembered I don't think its hooked up would be good to test. We don't need to add a http2error we just need to ensure when we get an h2 error its translated into the correct status code.

[davidpdrsn]

Is this related to #612?

[davidpdrsn]

It isn't. But it was fixed in #509