Rework exit model

While updating the confd code, I realised that we have _lots_ of
duplication in the exit model for the various programs.

So this patch attempts to abstract all the exits via a couple of new
functions; sorry for the somewhat big patch, but I hope the payoff is
worth the change: the actual exit conditions are much clearer.

Note that the patch (also) moves the exitIfBad function to Utils.hs,
since that is more logical.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>