TpConnection methods: add sanity checks with g_return_if_fail
TpConnection methods: add sanity checks with g_return_if_fail

TpConnection methods should critically warn if called on a non-Connection,
and functions called on a TpDBusDaemon should behave similarly.

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib