Custom fail method

Hey. First of all I dig the gem, loving how simple it is. Not sure if it was intended for my type of use though. I'm building an API which requires authentication for certain requests, but a simple redirect to '/whatever_route' isn't quite what one would expect when developing against an API and authentication fails.

Maybe I'm just lacking the necessary Sinatra skills (yes I am new at this), but something like

def authentication_required
  status 403
  {"error" => "Authentication required."}.to_json
end

set :session_fail, lambda { authentication_required }

would be awesome. Of course the "authentication_required" method fails here as it doesn't have the status method. Any way to accomplish this? Would love to send a PR with documentation if you can help me figure this out. Cheers! :)

Start by tracing the existing session-failure handling behind set :session_fail and how that callback is invoked. Confirm whether the callback can access Sinatra response helpers; done means an unauthenticated request can return status 403 and the requested JSON body, with documentation if supported.