Fix three clip table export bugs.
Fix three clip table export bugs.

Two of the bugs were Windows-specific. One caused export to fail when
the Windows version of datetime.strftime received certain extended
format strings, like one including "%3f". The other caused export to
fail when the file to be written already existed. The third,
cross-platform bug caused the recent clip count measurement to fail when
a clip had no classification.
HaroldMills/Vesper,HaroldMills/Vesper,HaroldMills/Vesper,HaroldMills/Vesper,HaroldMills/Vesper