Escape user supplied data to prevent XSS

This prevents openid_error from being used an XSS vector. See here for
more details:

http://seclists.org/oss-sec/2014/q2/122
Fix for CVE-2008-4730 - XSS

See here for details:

http://www.securityfocus.com/archive/1/archive/1/496930/100/0/threaded