rails and uploadify onError ajax call redirects to undefined action

heres my controller action:

  def create
        #logger.debug("Time.now is #{Time.now}")
        new_params = coerce(params)
        logger.debug new_params.inspect
        params[:user] = new_params[:user] 
        @user = User.api_create(params[:user])
        logger.info @user.inspect
       # redirect_to @user
        respond_to do |format| 
          if @user.save
            format.html {redirect_to @user} 
            format.json {render :json => { :user => user_path(@user) } }
          else
            format.html {redirect_to :controller => "application", :action => "index"} 
            format.json {render :json => {开发者_如何学C :errors => "errors" } }
          end
        end
      end

When I have no errors, the user path is returned to uploadify onComplete. But when I try to pass a string back to onError I get redirected to

ActionController::RoutingError (No route matches "/undefined"):

Rendered /Users/ddayan/.rvm/gems/ruby-1.9.2-p136/gems/actionpack-3.0.3/lib/action_dispatch/middleware/templates/rescues/routing_error.erb within rescues/layout (1.8ms)

Any idea what am I doing wrong??

Edit: Ok, I didn't think it through, if I render json than it goes to onComplete. So my new question is how do I return the error message to onError????

Edit2: So I changed the format to JS

format.js { render :text => "ERROR", :status => 403 }

I get the error status in the info field inside errorObj, but I don't have responseText

Object
  info: 403
  type: "HTTP"
  __proto__: Object