make sure to raise exception if redis is not there (#674)

### What problem does this PR solve?

### Type of change

- [x] Refactoring

api/db/services/task_service.py

@@ -159,4 +159,4 @@ def queue_tasks(doc, bucket, name):
     DocumentService.begin2parse(doc["id"])
 
     for t in tsks:
-        REDIS_CONN.queue_product(SVR_QUEUE_NAME, message=t)
+        assert REDIS_CONN.queue_product(SVR_QUEUE_NAME, message=t), "Can't access Redis. Please check the Redis' status."