Use either g_get_real_time() or g_date_time_new_now_local(). This means
we don’t need to worry about time_t being 32b in future (the year 2038
problem), and it makes the need for error handling a bit more explicit.
Improve the error handling in several cases.

Based on a patch by Niels De Graef
(https://gitlab.gnome.org/GNOME/glib/merge_requests/142).

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://gitlab.gnome.org/GNOME/glib/issues/1402