Update email content for failed payments

https://github.com/3scale/porta/blob/master/app/views/emails/invoice_messenger_unsuccessfully_charged_for_buyer.text.liquid

New message:

Dear {{ account.name }},

Thank you for using our service.

We're sorry to inform you that your last payment was declined.
This may have been caused by a few common reasons:

• A new authentication policy enforced by your bank
• An expired credit card
• Insufficient funds on the account

To continue using your service, verify the status of your credit card and update or re-enter the credit card details on the Account tab of your Dashboard.

If you need help, don't hesitate to contact us at {{ provider.finance_support_email }}.

Best regards,

The {{ provider.name }} API Team