serverutil/frontend: don't serve downloads page when no config present

We plan to start hosting Upspin releases by another mechanism, and so
make it possible to run the frontend without the downloads page by
not providing an upspin config.

Change-Id: Icc51d83ee8bf6d060156d5a948afa7a9556e0bf5
Reviewed-on: https://upspin-review.googlesource.com/c/upspin/+/19743
Reviewed-by: adg@google.com <adg@google.com>

serverutil/frontend/frontend.go

@@ -24,6 +24,7 @@ import (
 	"github.com/russross/blackfriday"
 
 	"upspin.io/config"
+	"upspin.io/errors"
 	"upspin.io/flags"
 	"upspin.io/log"
 	"upspin.io/serverutil/web"
@@ -53,7 +54,9 @@ func Main() {
 		})
 	} else {
 		cfg, err := config.FromFile(flags.Config)
-		if err != nil {
+		if errors.Is(errors.NotExist, err) {
+			log.Info.Printf("Upspin config not found; running without downloads page.")
+		} else if err != nil {
 			log.Fatal(err)
 		}
 		s, err := newServer(cfg, *docPath)