stbt batch report: Ignore invalid values in "extra-columns"

The extra-columns file can contain user-provided data (from the user's
pre-run, post-run, or classify hooks) so `stbt batch report` shouldn't
die because of an invalid value in one testrun (`stbt batch report`
reads the extra-columns information for every testrun, when generating
the summary html report).
stbt batch report: Show jpegs in testrun html view

`stbt batch run` doesn't generate jpegs, only pngs, but the user's
own test script, or pre- or post-run hooks, might do.
stbt batch report: Don't show "thumbnail.jpg" among test-run logs

Commit d220b984 started saving a thumbnail.jpg for each test run.
It's used as the thumbnail for the video. We don't want it listed
as a link alongside stdout.log etc.