Force having a valid ntp peer before we continue

I'm not sure what is causing the odd behaviour I'm seeing, but I'm suspicious that we are starting with "old time" (from fake-hwclock) sometimes which causes bad behaviour.  I'm not yet sure how much to trust MoPi's timing when sleeping for longer times (1/2 hour or even all night).
More logging improvements

Log actions (in particular knowing if we think we are taking a photo would be useful!)

Fix calculation of time skew to avoid getting datetime.timedelta(seconds=-0.000001)=-1 day, 23:59:59.999999, when we really want -0.00000001s
Copy syslog to USB