Mailchimp unsubscribe doesn't check if the user is already on the list

[sminnee]

This code routinely throws an error: https://github.com/camfindlay/silverstripe-apes/blob/1.0/code/APES.php#L196

It throws an error of the form:

There is no record of the email address "(email)" in your account

The problem is that it doesn't check to see whether the user is actually in the list before unsubscribing. This code should be amended to first check to see if the user is in the list, before unsubscribing.