Make the "invalid authenticity token" 403 error pretty like the others

- Users *shouldn't* see this, so don't include any extra information.

![screen shot 2014-12-18 at 12 16 34](https://cloud.githubusercontent.com/assets/355033/5487962/c3702a58-86af-11e4-8a07-c7c2f1440051.png)

`Invalid authenticity token` isn't the only reason we might issue a 403/call this method. 403 is "Forbidden", http://en.wikipedia.org/wiki/List_of_HTTP_status_codes#4xx_Client_Error

It might be easier to call `error_response(403, "Invalid authenticity token")` from the `rescue_from` block.