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)

I know you weren't the one to first add this, but should it actually be a `PUT` or a `PATCH`? (is it replacing all fields?)