iter: propagate runtime.Goexit from iterator passed to Pull

iter: propagate runtime.Goexit from iterator passed to Pull

This change propagates a runtime.Goexit initiated by the iterator into
the caller of next and/or stop.

Fixes #67712.

Change-Id: I5bb8d22f749fce39ce4f587148c5fc71aee2af65
Reviewed-on: https://go-review.googlesource.com/c/go/+/589137
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: David Chase <drchase@google.com>