event server: minor cleanup: return 0 from set_exposure as the doc says
event server: minor cleanup: return 0 from set_exposure as the doc says

OpenPHDGuiding/phd2,OpenPHDGuiding/phd2,OpenPHDGuiding/phd2,OpenPHDGuiding/phd2,OpenPHDGuiding/phd2,OpenPHDGuiding/phd2