girwriter: Report error on secondary top-level namespace to be written
girwriter: Report error on secondary top-level namespace to be written

Fixes https://gitlab.gnome.org/GNOME/vala/issues/805

frida/vala,GNOME/vala,GNOME/vala,frida/vala,GNOME/vala,frida/vala,GNOME/vala,frida/vala,frida/vala