[#127] Errors during app setup

https://github.com/jayroh/thredded_app/issues/127

* @StealthyGecko found the issue where an improperly filled in form
  during setup will error out with a "template missing error"
* The controller needed to re-render :new, display an error, and ensure
  that the previously populated fields persist.
* Failing tests written, now passing
* Updated zeus because wtf why not