Add exception handling for download data responses failure (#271)

lms/djangoapps/courseware/user_state_client.py

@@ -12,6 +12,7 @@
 import six
 from django.conf import settings
 from django.contrib.auth.models import User
+from django.core.exceptions import ObjectDoesNotExist
 from django.core.paginator import Paginator
 from django.db import transaction
 from django.db.utils import IntegrityError
@@ -425,6 +426,15 @@ def iter_all_for_block(self, block_key, scope=Scope.user_state):
                 if state == {}:
                     continue
 
+                try:
+                    sm.student
+                except ObjectDoesNotExist:
+                    log.warning(
+                        'The related user in student module record with the student_id %s does not exist',
+                        sm.student_id
+                    )
+                    continue
+
                 yield XBlockUserState(sm.student.username, sm.module_state_key, state, sm.modified, scope)
 
     def iter_all_for_course(self, course_key, block_type=None, scope=Scope.user_state):