Fix call to run_web_txn.pl for older msys systems.

This was broken way back at commit ecca968f. It's now of largely
historical interest, so I'm not going to push a release just to push the
bug fix.
Use longer form exit logic in run_web_txn.pl

I have no idea why this makes a difference, but it apparently does.
Not worth triggering a release over for the same reason as the last
commit.