Make many gun_http2 functions return state or error tuple

[zuiderkwast]

The following functions used to return a state, but now return {state, State} or {error, Reason}:

• frame/5
• update_window/1,2
• maybe_ack_or_notify/2
• reset_stream/3
• push_promise_frame/7
• goaway/2
• maybe_send_data/6
• send_data/4
• send_data/6
• send_data_frame/4

---

This is incremental refactoring preparing for #243

[zuiderkwast]

Good morning Loïc. Are you busy?

I believe when this is merged, the remaining "handle send errors" will be a very small change.

[essen]

Sounds good. Yes if I don't get to this today it will be next Monday.

[zuiderkwast]

OK, great. The change is large but trivial, so I hope it is easy to review.

Then, I can help finishing the remaining 2.0 projects, if you know what you want with them. :-)

[essen]

@zuiderkwast Reviewed. There's at least one error and a couple consistency problems that should easily be resolved.

[essen]

Merged, thanks! I've also fixed the TLS tests that fail. Dialyzer will fail until the other PR is merged.