gresolver: Document that GResolver lists are non-empty on success

The documentation previously wasn’t clear about whether the GResolver
methods could return an empty list and no error. On balance, this seems
like a bad idea, and GResolver should commit to always return a
non-empty list, or an error (which should be G_RESOLVER_ERROR_NOT_FOUND
if the list would otherwise be empty).

https://bugzilla.gnome.org/show_bug.cgi?id=728776
gresolver: Ensure GThreadedResolver always sets an error if resolution fails

It was previously possible for GThreadedResolver to return an empty list
and no error in response to a g_resolver_lookup_by_name() call, if it
happened that all the addresses returned by getaddrinfo() could not be
converted from native addresses to GSocketAddresses.

Fix that by setting a G_RESOLVER_ERROR_NOT_FOUND if the returned list is
empty.

https://bugzilla.gnome.org/show_bug.cgi?id=728776