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.
6 jobs for <span class="ref-name">fix/error_page_unicode</span> in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-TestRunner2

07:16:48

running ERP5.UnitTest-Master.Medusa

2373:55:45

failed ERP5.UnitTest-TestRunner2

14:50:44

running SlapOS.Eggs.UnitTest-Master.Python3

2373:55:35

passed ERP5.CodingStyleTest-TestRunner2

00:22:17

failed ERP5.UnitTest-TestRunner2

01:10:18