`future::recover()` interface is cumbersome

[sean-parent]

recover is passed a future and always invoked (I think I drove this decision). In practice, this is a bit of a pain - I think I'd rather haverecover just take the exception_ptr (but this relates to the recover issue #32 with when_all - where some information is available). Also, related to promise issue #261, when using a promise or make_exceptional_future() it is unfortunate that you have to use try/catch to extract information about the original exception. This leads to exceptions as flow control. I would love to have some sort of hybrid exception/error-code but I don't know of a clean/simple way to do that.