net/http, math/big, cmd/internal/gc/big: replaced errors.New(fmt.Sprintf(...)) in favour fmt.Errorf()

Change-Id: I38fc0ab84a374cb9be0234e40665d7cea0e76fc1
Reviewed-on: https://go-review.googlesource.com/8402Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>