Unhandled PG::UniqueViolation for topic external_id field

Great! I’ll see what I can do.

It probably doesn’t need to be as specific as what I was proposing yesterday:

rescue TopicCreator::DuplicateExternalIdError => e
   @errors.add(:base, "External ID must be unique. Existing topic ID: #{e.topic.id}")

Just getting a response with details about the what constraint was violated should be good enough.