If pre-flight checks don't pass, shut down more gracefully


git-svn-id: https://svn.calendarserver.org/repository/calendarserver@13925 e27351fd-9f3e-4f54-a53b-843176b1656c
calendarserver_diagnose


git-svn-id: https://svn.calendarserver.org/repository/calendarserver@13926 e27351fd-9f3e-4f54-a53b-843176b1656c