Update CheckMap.hs to make missing back link to lobby an ERROR

lib/CheckMap.hs

@@ -93,7 +93,7 @@ runLinter isMain config tiledmap depth = MapResult
   { mapresultLayer = invertThing layer
   , mapresultTileset = invertThing tileset
   , mapresultGeneral =
-    ([Hint Warning "main.json should link back to the lobby" | isMain && not (any linksLobby layerDeps)])
+    ([Hint Error "main.json should link back to the lobby" | isMain && not (any linksLobby layerDeps)])
     <> lintsToHints (resultToLints generalResult)
   , mapresultDepends = resultToDeps generalResult
     <> layerDeps