Fix error in unsubscriber

- wrong error checking

cmd/hubserver/handler.go

@@ -87,38 +87,47 @@ func (handler *subscriptionHandler) handleUnsubscription(w http.ResponseWriter,
 	topic := r.Form.Get("hub.topic")
 	mode := r.Form.Get("hub.mode")
 
-	if subscribers, err := handler.store.Subscribers(topic); err != nil {
-		for _, subscriber := range subscribers {
-			if subscriber.Callback != callback {
-				continue
-			}
-			ourChallenge := randStringBytes(12)
+	subscribers, err := handler.store.Subscribers(topic)
+	if err != nil {
+		return errors.Wrapf(err, "could not get subscribers for %s", topic)
+	}
 
-			validationURL, err := url.Parse(callback)
-			if err != nil {
-				log.Println(err)
-				return err
-			}
-			q := validationURL.Query()
-			q.Add("hub.mode", mode)
-			q.Add("hub.topic", topic)
-			q.Add("hub.challenge", ourChallenge)
-			validationURL.RawQuery = q.Encode()
-			if validateURL(validationURL.String(), ourChallenge) {
-				err = handler.store.Unsubscribe(topic, callback)
-				if err != nil {
-					return err
-				}
-			}
+	found := false
+	for _, subscriber := range subscribers {
+		if subscriber.Callback != callback {
+			continue
 		}
 
-		w.WriteHeader(200)
+		found = true
+
+		ourChallenge := randStringBytes(12)
+
+		validationURL, err := url.Parse(callback)
+		if err != nil {
+			log.Println(err)
+			return err
+		}
+		q := validationURL.Query()
+		q.Add("hub.mode", mode)
+		q.Add("hub.topic", topic)
+		q.Add("hub.challenge", ourChallenge)
+		validationURL.RawQuery = q.Encode()
+		if validateURL(validationURL.String(), ourChallenge) {
+			err = handler.store.Unsubscribe(topic, callback)
+			if err != nil {
+				return err
+			}
+		}
+	}
+
+	w.WriteHeader(200)
+	if found {
 		_, err = fmt.Fprintf(w, "Unsubscribed\n")
-		return err
 	} else {
 		http.Error(w, "Hub does not handle subscription for topic", 400)
 	}
-	return nil
+
+	return err
 }
 
 func (handler *subscriptionHandler) handleSubscription(w http.ResponseWriter, r *http.Request) error {