MAPG-235 challenge token related error handling

src/Controller/GameController.php

@@ -107,6 +107,10 @@ class GameController
     {
         // create Challenge
         $challengeToken = rand();
+        while ($this->challengeRepository->getByToken($challengeToken)) {
+            // if a challenge with the same token already exists
+            $challengeToken = rand();
+        }
 
         $challenge = new Challenge();
         $challenge->setToken($challengeToken);