Redirect to referer on Error #8

If `onError` is not set, the default referer will be used.

src/main/java/de/nclazz/service/mailrelay/adapter/web/MessageFormController.java

@@ -29,7 +29,8 @@ public class MessageFormController {
                                  BindingResult bindingResult) {
 
         if(bindingResult.hasErrors()) {
-            return "redirect:" + form.getOnError();
+            String onError = (form.getOnError() != null ? form.getOnError() : referer);
+            return "redirect:" + onError;
         }
 
         String token = form.getToken();