error messages for missing env vars

main.go

@@ -25,6 +25,9 @@ func onReconnecting(client MQTT.Client, clientOpts *MQTT.ClientOptions) {
 
 func onConnect(client MQTT.Client) {
   topicsStr := os.Getenv("MQTT_SUBSCRIBE_TOPICS")
+  if topicsStr == "" {
+    log.Fatal("No topics given, set env var MQTT_SUBSCRIBE_TOPICS")
+  }
   topics := strings.Split(topicsStr, ",")
   for _, topic := range topics {
     if token := client.Subscribe(topic, 0, onMessageReceived); token.Wait() && token.Error() != nil {
@@ -42,6 +45,9 @@ func main() {
   log.Println("UDI starting")
 
   broker := os.Getenv("MQTT_BROKER")
+  if broker == "" {
+    log.Fatal("No broker given, set env var MQTT_BROKER")
+  }
 
   prefix := "UDI"
   uuid := uuid.New()