Send an email when IPN errors are received from Paypal

This pull request is related to this feature: `When the Seamless Donations IPN receives a "Subscription cancelled" or a "Payment error" event from Paypal, send a notification email to a predefined CS email address. `

Now we will receive an email for all IPN notifications which are not a "payment_complete" notification. This includes: errors when processing a payment, a cancelled subscription, ...

In the future we can fine tune these notifications, so we trigger custom emails for each of the different cases.

What do you think?