The patches make good sense to me -- I have no comments to add in a review.

I spent more time than I care to admit concerned with the idea that error_callback (exposed by map_async which map sits on top of) should perhaps be called not just once at the end but each time an exception occurs.  Motivated by past jobs which failed overall to yield any results because one out of a million of the inputs triggered an error, I thought the idea very appealing and experimented with implementing it (with happy results).  Googling for it though, I found plenty of examples of people asking questions about how callback and error_callback are intended to work -- though the documentation is not explicit on this particular point, most of those search results correctly document in the wild that error_callback is called only once at the end just like callback.  I think it best to leave that functionality just as you have it now.

Thanks for creating the patch -- looks great to me.