Reformat zippy responses to do the right thing (bug 948240)

https://bugzilla.mozilla.org/show_bug.cgi?id=948240

I added the `resource_pk` → `id` conversion + standard `error_message` key in case of exception. Not sure what else to add given that we plan to remove all lists due to https://bugzilla.mozilla.org/show_bug.cgi?id=957632 @andymckay @kumar303?