Add Q_REQUIRED_RESULT to force callers to check the return; the
QTranslator object is unusable if load() fails.
Check the result in QTranslator's own test.
Task-number: QTBUG-85700
Change-Id: I07509c76470cc87626190670665cd3162bfb17e7
Reviewed-by: Lars Knoll <lars.knoll@qt.io>