Error handleing with redirect_to

I'm sketching out a error handler, something like

if cando = false
   redirect_to( :controller => Oops,
                :action => 'accessdenied',
                :type => 'alarm',
                :message => 'no_permission_to_view',
                :return_to => saved_url,

Is this an internal redirection or does it immedate send that url to the
browser as


What I want is the message sent to the user with a view custom for the
action parameter. It would be nice it if it was a popup rather than a whole
new page, but I'll live with that.

Am I on the wrong track here?
I seem to be getting problems with routing.