Handle failure properly in rust_sched_loop::schedule_task

[catamorphism]

There's a commented-out assertion with a FIXME: "in the face of failing tasks, this is not always right."

[bblum]

I'd take it, but a look over doesn't immediately reveal why. @brson ?

[brson]

That FIXME is ancient, referring to an invariant maintained by a commented-out assert that references a variable that no longer exists. I think it is just totally bogus now.