Auth_Default: Add support for exception handling.

This function makes SimpleSAML_Auth_Default save the errorURL given
to the login functionin the state array. This will make the
SimpleSAML_Auth_State class pass exceptions to this url.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1572 44740490-163a-0410-bde0-09ae8108e29a