set exceptions on join group

added emoji to error.html <- wichtig

                                                          Co-authored-by: AndiBuls <andreas.buls@uni-duesseldorf.de>

src/main/java/mops/gruppen2/controller/MopsController.java

@@ -1,5 +1,7 @@
 package mops.gruppen2.controller;
 
+import mops.gruppen2.domain.exception.EventException;
+import mops.gruppen2.domain.exception.PageNotFoundException;
 import org.springframework.stereotype.Controller;
 import org.springframework.web.bind.annotation.GetMapping;
 
@@ -20,7 +22,7 @@ public class MopsController {
     }
 
     @GetMapping("*")
-    public String defaultLink() {
-        return "error";
+    public String defaultLink() throws EventException {
+        throw new PageNotFoundException("\uD83D\uDE41");
     }
 }