cmd/upspin-setupstorage-dropbox: if oauth2 exchange fails exit setup with an error

Change-Id: I83f6a5a09b78a8ed1002fdba79119e9ca1d42168
Reviewed-on: https://upspin-review.googlesource.com/c/19240
Reviewed-by: Rob Pike <r@golang.org>
diff --git a/cmd/upspin-setupstorage-dropbox/setupstorage.go b/cmd/upspin-setupstorage-dropbox/setupstorage.go
index 68487dc..9603008 100644
--- a/cmd/upspin-setupstorage-dropbox/setupstorage.go
+++ b/cmd/upspin-setupstorage-dropbox/setupstorage.go
@@ -89,7 +89,7 @@
 
 	token, err := conf.Exchange(oauth2.NoContext, code)
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "error in fetching oauth2 token: %v.\n", err)
+		s.Exit(err)
 	}
 
 	return token.AccessToken