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

LewisHaley/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester