stbt batch: Extract backtrace from log and show in testrun html page

This reinstates (in spirit) commit 98c191db (stbt batch: Show backtrace
in testrun html page), which was reverted because it's a bit of a hack
and we were expecting to use pytest by now. However pytest support is
taking longer than expected, so I'm reinstating this for now.
stbt batch run: Don't run test if the pre-run script fails
Revert "stbt batch run: Don't run test if the pre-run script fails"

This reverts commit e25bd92c5ea34711b0f9879b49472c93b76cf63b.

This leaves around a half-baked testrun directory, which `stbt batch
report` doesn't like (I think). Reverting for now; when I have more time
I'll look into it properly, with unit tests to make sure it's working,
etc.