Fix test checking for nested exception
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]: https://github.com/celery/celery/commit/32b52ca875509b84d786e33ce2d39f62ab7ea050#diff-db604dd7cb51e386710260ff2eba378aac19ba11eec97904bbf097b68caeada6L625

alphagov/notifications-api,alphagov/notifications-api