prevent errors to be stored in game cache

By just replacing them with the absence of a game.
This will cause the round and proxy to be terminated,
and the next request will try to load it again.

modules/round/src/main/GameProxy.scala

@@ -97,7 +97,7 @@ final private class GameProxy(
 
   private[this] var cache: Fu[Option[Game]] = fetch
 
-  private[this] def fetch = gameRepo game id
+  private[this] def fetch = gameRepo game id recoverDefault none
 }
 
 private object GameProxy {