gtimezone: Add new constructor which can report errors

Philip Withnallrequested to merge
pwithnall/glib:553-tz-errors into master

Add a new variant of g_time_zone_new() which returns NULL on failure to load a timezone, rather than silently returning UTC.

Signed-off-by: Philip Withnall pwithnall@endlessos.org

Fixes: #553 (closed)

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