提前初始化updatelist，fix找不找变量的问题

backend/SystemUpdater/UpdateManager.py

@@ -198,6 +198,8 @@ class UpdateManager():
                     "message:\n") + str(e)
             _success = False
             return _success,header,desc
+            
+        self.update_list = UpdateList(self)
 
         #更新important.list
         if self.options.no_update_source is False:
@@ -210,7 +212,6 @@ class UpdateManager():
         if _success == False:
             return _success,header,desc
 
-        self.update_list = UpdateList(self)
         try:
             _success,header,desc = self.update_list.update(self.cache)
         except SystemError as e: