gnutls: ensure connection never fails without an error
gnutls: ensure connection never fails without an error

This regressed in 18360afb03b5b23887f6f027bddb4f19c9fc4be8, when I
carelessly added a codepath that allows initialization to fail without
an error.

GNOME/glib-networking,GNOME/glib-networking,GNOME/glib-networking