Add UnavailableException for alert checkers
There is a problem with alerts like NTP one, those which can't be checked right now for various reason (e.g. system was started too recently). If we reboot system that had such an alert, after it boots we'll receive a notification that alert is gone; however, most likely it is not. I think that checkers can raise some
UnavailableException that will leave alerts in their current state.