Fix test checking for nested exception

Previously this type of exception was raised at the top level and
the task did not retry [1]. Since Celery 4+ the behaviour changed
so that a Retry exception will be raised unless we explicitly say
we want to raise the original one [2].

It's unclear if we actually want to retry this task for any type
of exception, but it's out-of-scope for this PR to decide on this,
so here we just reraise the exception to make it compatible with
the new version of Celery and the existing test.

[1]: https://github.com/alphagov/notifications-api/pull/2832/files#diff-926badba91648d56a973e16bd92da3345b23bc60dc89360119b1df08de52723fL77
[2]: 32b52ca875 (diff-db604dd7cb51e386710260ff2eba378aac19ba11eec97904bbf097b68caeada6L625)

app/celery/letters_pdf_tasks.py

@@ -85,12 +85,12 @@ def get_pdf_for_templated_letter(self, notification_id):
             args=(encrypted_data,),
             queue=QueueNames.SANITISE_LETTERS
         )
-    except Exception:
+    except Exception as e:
         try:
             current_app.logger.exception(
                 f"RETRY: calling create-letter-pdf task for notification {notification_id} failed"
             )
-            self.retry(queue=QueueNames.RETRY)
+            self.retry(exc=e, queue=QueueNames.RETRY)
         except self.MaxRetriesExceededError:
             message = f"RETRY FAILED: Max retries reached. " \
                       f"The task create-letter-pdf failed for notification id {notification_id}. " \