In a given program there may be two separate copies of
ErrNoCachedConn: the h2_bundle.go version in net/http, and the user's
golang.org/x/net/http2 version. We need to be able to detect either
in net/http.

This CL adds a function to report whether an error value represents
that type of error, and then a subsequent CL to net/http will use it
instead of ==.

Updates golang/go#22091

Change-Id: I86f1e20704eee29b8980707b700d7a290107dfd4
Reviewed-on: https://go-review.googlesource.com/87297Reviewed-by: Tom Bergan <tombergan@google.com>