stbt batch run: Only save screenshot if `stbt run` didn't already save one

`stbt run` will save a screenshot if the test script raises an uncaught
exception with a "screenshot" member containing a video frame.

In the dawn of time, the screenshot saved by `stbt run` would contain
the visualisation from `wait_for_match` (a red or yellow rectangle) or
from `wait_for_motion` (red pixels), so we'd take a separate
"screenshot-clean.png" always. But for quite a long time now the
screenshot saved by `stbt run` doesn't contain that visualisation, so
there isn't any need for two separate screenshots.
stbt batch report: Fix error when testrun dir name has spaces

This can happen if you use `stbt batch run -t "something with spaces"`.
The error was: "dirname: extra operand `with'".
It stopped the report from being generated, though the html for the
individual testrun details was still generated correctly. And you could
generate the summary report by manually running `stbt batch report
index.html`.