gsocket: Fix error behaviour of g_socket_send_messages()

If an error in the underlying sendmmsg() syscall occurs after
successfully sending one or more messages, g_socket_send_messages()
should return the number of messages successfully sent, rather than an
error. This mirrors the documented sendmmsg() behaviour.

This is a slight behaviour change for g_socket_send_messages(), but as
it relaxes the error reporting (reporting errors in fewer situations
than before), it should not cause problems.

https://bugzilla.gnome.org/show_bug.cgi?id=751924