It seems like we just drop the exception stack when a promise resolve/reject callback throws. In particular MaybeGetAndClearException just ignores the exception stack. Instead we should probably propagate that stack through various Promise methods.

Compare

js> Promise.reject("foobar")
js> quit()
Unhandled rejection: "foobar"
Stack:
  @typein:1:9

and

js> Promise.resolve().then(() => { throw "foobar"; })
js> quit()
Unhandled rejection: "foobar"
(no stack trace available)

We already have the pending exception stack, we might as well use it.
This helps in cases where we reject the promises but the current stack is already empty again.

https://hg.mozilla.org/mozilla-central/rev/2b2753c09568