Add --fail-fast

[lgarrison]

This adds the --fail-fast option to exit promptly if any task fails. It seems to work, but I have low confidence that this is the right implementation. In particular, engines with pending tasks fail currently, rather than gracefully closing. I'm not even 100% sure what the desired semantics are—do we want to wait for running tasks to finish, or kill them immediately?

[njcarriero]

Looks straightforward. The only thing I would recommend is adding a comment (which I should have done in the code fragment on which I am guessing this is modeled, around 1580) to the effect that "break" at 1532 in this context breaks out of the main driver control loop and drops into the exit code.

Thanks!

[lgarrison]

I wasn't sure whether to be worried about the fact that disBatch exits with a non-zero exit code when failing fast (along with the message Some engine processes failed -- please check the logs). Normally a task failure would only cause a non-zero exit code with -e. But the current fail-fast behavior is to interrupt running tasks, so maybe the user does want to be notified that this has happened.

I think what I'm leaning towards is keeping the exit code and probably the Some engine processes failed message, and adding one more stderr message that says disBatch quit early due to task failure with --fail-fast or something like that. With just the former error message, it looks like an internal failure unless one goes spelunking in the logs for the "fail fast" message.