UI should job.MISSING in sequential job case

[robnagler]

There's very complicated code in the supervisor to handle the case that a sequential job has been purged. Rather, return missing (just like parallel case), and have the UI automatically resend. The only reason this code needs to be here is that sequential (transient) simulations do not have a button to start them, which means the onus is on the UI to resend, not the supervisor.

[robnagler]

@moellep if you could handle the UI part, that would be great. You can test by editing out special case, but don't commit those changes. I'm working on all that code, and it's quite messy, so would be better to not have merge issues. Adding to the UI before the supervisor change is a no op so should be safe to merge your changes before I get my PR done.