there are no responsecodes for subscriptions you are right, but you should register webhooks for this. Than you'll be informed if a transaction of this subscription fails and also gets the errorcode. Please have a look for webhooks here https://www.paymill.com/en-gb/documentation-3/reference/api-reference/#webhooks.
We will also make a refactoring for subscription in the first quarter of 2014. Than we'll improve also the responses.
Best, Christian