TpBaseContactList: add pre-condition checks to match documentation
TpBaseContactList: add pre-condition checks to match documentation

Guillaume noticed during review that various methods documented that it
was an error to call the method before the contact list had been
retrieved, but didn't actually check it.

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