restart the auth flow when the facebook session expired

app.rb

@@ -2,6 +2,8 @@
 require "mogli"
 
 enable :sessions
+set :raise_errors, false
+set :show_exceptions, false
 
 FACEBOOK_SCOPE = 'email, status_update, publish_stream, publish_actions, user_likes, user_photos'
 
@@ -32,6 +34,12 @@ def first_column(item, collection)
   end
 end
 
+# the facebook session expired! reset ours and restart the process
+error(Mogli::Client::HTTPException) do
+  session[:at] = nil
+  redirect "/auth/facebook"
+end
+
 get "/" do
   redirect "/auth/facebook" unless session[:at]
   @client = Mogli::Client.new(session[:at])