s4-python: fixed annoyance where control-C doesn't kill our python scripts

s4-python: fixed annoyance where control-C doesn't kill our python scripts

We want our scripts to die immediately when a user hits
control-C. Otherwise we not only annoy the hell out of the user, we
also risk db corruption as the control-C could get delivered as an
exception which gets mis-interpreted (eg. as a missing db object). We
use transactions for all our databases, so the right thing to do in
all our command line tools is to die immediately.