Make the glib-networking dependency more explicit

Make the glib-networking dependency more explicit

Require glib-networking to be present at build time (with an error
explaining that you can pass --disable-tls-check, but packagers should
make sure libsoup has a runtime dependency on glib-networking).

Also, if a TLS connection fails because of missing glib-networking,
mention that in the reason_phrase, as a last resort.