[release-branch.go1.13] net/http: don't cache http2.erringRoundTripper connections

Fixes #35087
Updates #34978

Change-Id: I3baf1392ba7366ae6628889c47c343ef702ec438
Reviewed-on: https://go-review.googlesource.com/c/go/+/202078


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit 88186e5e)
Reviewed-on: https://go-review.googlesource.com/c/go/+/202642


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>