[release-branch.go1.15] net/http: fix detection of Roundtrippers that always error

CL 220905 added code to identify alternate transports that always error
by using http2erringRoundTripper. This does not work when the transport
is from another package, e.g., http2.erringRoundTripper.
Expose a new method that allow detection of such a RoundTripper.
Switch to an interface that is both a RoundTripper and can return the
underlying error.

Fixes #45076.
Updates #40213.

Change-Id: I170739857ab9e99dffb5fa55c99b24b23c2f9c54
Reviewed-on: https://go-review.googlesource.com/c/go/+/243258


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/304210


Trust: Dmitri Shuralyov <dmitshur@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>