The idea is that this code silences the ALREADY_SUBSCRIBED error to make `add_subscriber` idempotent.

`process_result` can handle the other error cases.

It is my understanding that `response.results` is a list of responses given the list of data you provide to the API.

For example, `ET_TriggeredSend` can take a list of subscribers to send the email to and there is one entry in the results list for each subscriber.

We never try and send an email to more than one address at a time so the results list should always have one item.