create_xml_stream_parser(): Don't return a glib::Error; use a custom error code
create_xml_stream_parser(): Don't return a glib::Error; use a custom error code

The only error here is "could not create the libxml2 parser", so add
another variant to ParseFromStreamError for that.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg