add error cube, to show when model is missing

src/defs.txt

@@ -23,6 +23,8 @@ actor -2265 10 0 "suit"
     alive "yes"
     pronoun "it"
 
+actor 0 0 0 "error"
+
 chat "error"
     name "ERROR"
     msg 0 "INIT" "EXIT" "Unspecified conversation ID"

src/world.rs

@@ -25,7 +25,7 @@ fn asset_name_to_path(name: &str) -> &'static str {
         "asteroid1" => ASSET_ASTEROID1,
         "asteroid2" => ASSET_ASTEROID2,
         "pizzeria" => "models/pizzeria.glb#Scene0",
-        _ => "external/alien.glb#Scene0",
+        _ => "models/error.glb#Scene0",
     }
 }