mlprof bug

[Matthew Fluet]

> Anyways, as far as the basis library is concerned, this is all correct.
> But, mlprof fails with the unhelpful message
> syserr: xdev: Invalid ???   --- damn, I forget exactly.

The message is:
error: xdev: Invalid cross-device link

> What seems a little strange was that wasn't an unhandled exception, and I
> couldn't find a handler that would print out that type of message and
> exit.

I just didn't look hard enough; it's handled in 
(src/lib/mlton/basic) Process.makeMain.