fix(tonic): Expose h2 error instead of reason

[roblabla]

Motivation

Currently, when an error occurs at the H2 layer, it is transformed into a tonic::Status with a source() containing only the H2 error reason, instead of the full error. This loses a ton of context, as it is now impossible to know if the failure was caused by a protocol error, a general IO error (and if so, what what IO error), or something else entirely. It makes it much harder to debug problems.

It is unclear why this choice was made in #612. There's talk about Clone implementations causing this to be necessary, but I can't find that implementation.

Solution

This MR simply bubbles up the underlying h2::Error. tonic::Status::source will contain an h2::Error instead of an h2::Reason.

[davidpdrsn]

iirc Status only contained the h2 error reason, since Status needed to be Clone. However when we removed that requirement in #612 I think we just forgot to store the whole error instead of only the reason.

I think this change makes sense but its probably a breaking change since source downcasts will have to change.