Dead lettering is done on a best effort basis. The same
message might be dead lettered multiple times.
If validation on any of the fields fails at subscription
creation/updation, the create/update subscription request will fail.
.. attribute:: dead_letter_topic
The name of the topic to which dead letter messages should be
published. Format is projects/{project}/topics/{topic}.The
Cloud Pub/Sub service account associated with the enclosing
subscription's parent project (i.e.,
service-{project_number}@gcp-sa-
pubsub.iam.gserviceaccount.com) must have permission to
Publish() to this topic. The operation will fail if the topic
does not exist. Users should ensure that there is a
subscription attached to this topic since messages published
to a topic with no subscriptions are lost.
