Compare letter page count with billable units in DVLA response file

We compare the page_count in the response file we receive from the DVLA
with the billable_units of the letter. If these don't match, we log an
error.

app/celery/tasks.py

@@ -45,6 +45,7 @@ from app.dao.notifications_dao import (
     dao_update_notifications_for_job_to_sent_to_dvla,
     dao_update_notifications_by_reference,
     dao_get_last_notification_added_for_job_id,
+    dao_get_notification_by_reference,
 )
 from app.dao.provider_details_dao import get_current_provider
 from app.dao.service_inbound_api_dao import get_service_inbound_api_for_service
@@ -476,6 +477,8 @@ def update_letter_notifications_statuses(self, filename):
         raise
     else:
         for update in notification_updates:
+            check_billable_units(update)
+
             status = NOTIFICATION_DELIVERED if update.status == DVLA_RESPONSE_STATUS_SENT \
                 else NOTIFICATION_TEMPORARY_FAILURE
             updated_count = dao_update_notifications_by_reference(
@@ -503,6 +506,16 @@ def process_updates_from_file(response_file):
     return notification_updates
 
 
+def check_billable_units(notification_update):
+    notification = dao_get_notification_by_reference(notification_update.reference)
+
+    if int(notification_update.page_count) != notification.billable_units:
+        msg = 'Notification with id {} had {} billable_units but a page count of {}'.format(
+            notification.id, notification.billable_units, notification_update.page_count)
+
+        current_app.logger.error(msg)
+
+
 @notify_celery.task(bind=True, name="send-inbound-sms", max_retries=5, default_retry_delay=300)
 @statsd(namespace="tasks")
 def send_inbound_sms_to_service(self, inbound_sms_id, service_id):