Error reporting behavior in promise-done

going off from #6

just fixing the error call in promise-done, might miss the actual point of the throw in js Promise.done

This throw in a setTimeout seems to me the best way to get platform-neutral error reporting in JS. The closest equivalent in emacs would be just a call to message, ~~with printing logic for errors~~ and possibly a conditional debug - breakpoint to have a closer look at the data.