ignoring JSON

This is probably a strange request...

I want use Better Errors for HTML requests in dev. If the request is JSON, whether dev or production, I want to emit a collection+JSON error.

Since I'm using Rails, I would love to handle this at the ApplicationController level. I see that Better Errors is hooked in at the rack level, so I suspect I can't use something like

```
def handle_exception( exception )
respond_to do |format|
format.html do
if( APP_CONFIG[:show_stacktrace] )
render :text => BetterError.for( exception ).to_html
else
render :file => 'public/404.html'
end
end
format.json { render :json => ExceptionEndpoint.new(exception).to_json }
end
end
```

FWIW, ExceptionEndpoint isn't going to expose anything sensitive.

Since this is a rack app, though, I'm thinking I'll have to hook in some other way. Any direction would be appreciated.