Synthesise status when stream closes unexpectedly

[gjcairo]

Motivation

When the stream closes unexpectedly (the H2 stream channel becomes inactive, an error is fired through the pipeline, or we receive a RST_STREAM frame), the client-side inbound sequence either finishes or throws an error. To make it easier for higher layers to retry, we want to synthesise a Status instead.

Modifications

• On the client side, write a Status when one of the aforementioned unexpected closures happens.
• On the server side, make sure we're always firing an error when closing unexpectedly. I'm not sure this is the right thing to do though.
• On both, make sure we transition the stream state machine to closed.

Result

Inbound sequence now returns a Status instead of finishing/throwing