Verify add-on existence (bugs 941538 & 957648)

https://bugzilla.mozilla.org/show_bug.cgi?id=941538
https://bugzilla.mozilla.org/show_bug.cgi?id=957648
Merge pull request #1605 from davidbgk/941538-addonpaymentaccount-DNE

Verify add-on existence (bugs 941538 & 957648)