xhtml_style: fix rendering of standard_error_message when site title is not ASCII

When rendering error page, the default header_title is computed in global_definitions
and this is done by concatenating string:ERP5 (which is unicode) and the
title of the portal (which is an UTF-8 encoded string), which causes an
UnicodeDecodeError, because the portal title was decoded as ASCII.

Because header_title is usually an UTF-8 encoded string, also use an UTF-8
encoded string for 'ERP5' default value.