Show in-app product errors inline (bug 1028389)

https://bugzilla.mozilla.org/show_bug.cgi?id=1028389


![In-app product errors](https://cloud.githubusercontent.com/assets/211578/3362006/08e12434-fb05-11e3-9329-995dcb258681.gif)

Ah, I tried that and it returned an array so I figured it wasn't what I wanted. Looks like you can pass a selector.