Feature request: set exit status when goal unreachable

[rpgoldman]

If one is going to chain together the parser, grounder, engine, and then parser again, it would be very helpful to know when an intermediate stage has failed.

So it would be very helpful if the grounder was modified so that its exit status is non-zero when it proves that the goal is unreachable.

Otherwise, to handle the pipeline one must parse the output of the grounder.

If you would prefer not to do this in general, maybe you could add a command line argument like --error-on-fail that we could add when we want the exit status to be set.

Thanks!