Comment 10 for bug 1258655

There is only one possible exception thrown from the offending mga::HWCCommonDevice::mode()

    if (err)
    {
        std::string blanking_status_msg = "Could not " +
            ((mode_request == mir_power_mode_off) ? std::string("blank") : std::string("unblank")) + " display";
        BOOST_THROW_EXCEPTION(
            boost::enable_error_info(
                std::runtime_error(blanking_status_msg)) <<
            boost::errinfo_errno(-err));
    }

Safe to say whatever the cause, failure to power off the screen should not be treated as a fatal exception. Perhaps powering on should, but even then we need to avoid the potential spurious errors of "the screen is already on".