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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
(!142).
Signed-off-by: Philip Withnall <withnall@endlessm.com>

#1402