celery.task.retry exc param should be a throwable.

This causes an issue when it hits the max retry limit, and tries to
throw your exception to let you deal with it - at this point it was
moaning because we pass in a string

if it's not defined, and we're inside an exception block celery uses
that instead.

app/celery/callback_tasks.py

@@ -13,6 +13,6 @@ def process_ses_results(self, response):
         errors = process_ses_response(response)
         if errors:
             current_app.logger.error(errors)
-    except Exception:
+    except Exception as exc:
         current_app.logger.exception('Error processing SES results')
-        self.retry(queue=QueueNames.RETRY, exc="SES responses processed with error")
+        self.retry(queue=QueueNames.RETRY)