Core: improve error message for missing "game" entry in yaml (ArchipelagoMW#4185)

Author: beauxq

Generate.py

@@ -453,6 +453,10 @@ def roll_settings(weights: dict, plando_options: PlandoOptions = PlandoOptions.b
             raise Exception(f"Option {option_key} has to be in a game's section, not on its own.")
 
     ret.game = get_choice("game", weights)
+    if not isinstance(ret.game, str):
+        if ret.game is None:
+            raise Exception('"game" not specified')
+        raise Exception(f"Invalid game: {ret.game}")
     if ret.game not in AutoWorldRegister.world_types:
         from worlds import failed_world_loads
         picks = Utils.get_fuzzy_results(ret.game, list(AutoWorldRegister.world_types) + failed_world_loads, limit=1)[0]