strings: Update gold info page link.
Remove unused GoldRequiredException.
Generalize wiki validator / error system.

* Generalize validator error class and make an Exception subclass.
* Provide per-controller handling of fatal validator errors.
* Instantiate correct http exception classes using status_map.
Cleanup.
Rename "error" param to be a bit more pedantic.

As we more further towards errors being actual objects.