[Bug fix][Refactor] Course scores & Download

- Fix a bug that scores in course mode cannot calculate rating
- Make some optimization for downloading songs

latest version/server/course.py

@@ -1,6 +1,5 @@
 from core.constant import Constant
 from core.course import UserCourseList
-from core.error import ArcError
 from core.item import ItemCore
 from core.sql import Connect
 from core.user import UserOnline