net/http/httputil: t.Error -> t.Errorf

Found by vet.

Change-Id: I09b79d68c7a5fc97e0edda4700a82bfbb00a4f45
Reviewed-on: https://go-review.googlesource.com/28486
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>

src/net/http/httputil/reverseproxy_test.go

@@ -342,7 +342,7 @@ func TestReverseProxyCancelation(t *testing.T) {
 	}()
 	res, err := http.DefaultClient.Do(getReq)
 	if res != nil {
-		t.Error("got response %v; want nil", res.Status)
+		t.Errorf("got response %v; want nil", res.Status)
 	}
 	if err == nil {
 		// This should be an error like: