add some sense of api error handling on web server

common/db.py

@@ -96,6 +96,9 @@ class Db:
             'grant_type': 'refresh_token',
             'refresh_token': refresh_token,
             })
+        if res.status_code != 200:
+            return None
+
         print(res.status_code)
         print(res.content)
         res = res.json()