provide templated error pages for subsite. without this, error pages for host-node-mapped subsites are a pain CVS: ----------------------------------------------------------------------
- Execute pacakge-specfic code on every page. Callbacks of this type typically call template::head::* functions to add application specific code (CSS and JavaScript) to every page (e.g. like e.g. the cookie-consent-plugin). This callback is a generalization of the callbacks "subsite::get_extra_headers" and "subsite::header_onload".