Item Entity: Add message to moveresult assertion (#9797)

builtin/game/item_entity.lua

@@ -200,7 +200,9 @@ core.register_entity(":__builtin:item", {
 			return -- Don't do anything
 		end
 
-		assert(moveresult)
+		assert(moveresult,
+			"Collision info missing, this is caused by an out-of-date/buggy mod or game")
+
 		if not moveresult.collides then
 			-- future TODO: items should probably decelerate in air
 			return