Modify copy on card failed email to reflect double charge

[rohitpaulk]

From gratipay/inside.gratipay.com#888 (comment):

I think that the logic fits. I assume we send a notification when a card has failed. Can we edit the copy so that it describes this behavior? ie "Hi friend! Please update your credit card information so that we can process your tips. We only process payroll once each week but if you update your information before the next processing you will see a charge for both week's tips at the same time. We will never try to process more than two weeks tips so if it takes you a little longer, no worries."