Add a couple more cases where we convert random network I/O errors
into errRequestCanceled if the request was forcefully aborted.

It failed ~1/1000 times without -race, or very easily with -race.
(due to -race randomizing some scheduling)

Fixes #11894

Change-Id: Ib1c123ce1eebdd88642da28a5948ca4f30581907
Reviewed-on: https://go-review.googlesource.com/18287Reviewed-by: Russ Cox <rsc@golang.org>