cmd/go/internal/modfetch: avoid path.Join in URL errors, part 2
CL 406675 added more detail to bare errors from net/http in two places.
CL 461682 improved one of the two places to stop folding "//" into "/".
This CL applies the same change to the other place.
For #52727.
Change-Id: I3fc13f30cf0f054949ce78269c52b7fafd477e70
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/532015 Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>