Repairs a bug in `drain_to_final_msg()` where in the `Yield()` case
block we weren't guarding against the `ctx._stream is None` edge case
which should be treated a `continue`-draining (not a `break` or
attr-error!!) situation since the peer task maybe be continuing to send
`Yield` but has not yet sent an outcome msg (one of
`Return/Error/ContextCancelled`) to terminate the loop. Ensure we
explicitly warn about this case as well as `.cancel()` emit on a taskc.
Thanks again to @guille for discovering this!
Also add temporary `.info()`s around rxed `Return` msgs as part of
trying to debug a different bug discovered while updating the
context-semantics test suite (in a prior commit).
fde6d84b9d