Add a reason enum to HTLCDestination in some variants #3561
Description
It'd be nice if it was explicit why we failed when giving the user a HTLCHandlingFailed - HTLCDestination provides that for many cases, but the NextHopChannel case (and maybe others) can happen for different reasons. It'd be nice if we were explicit about that. Needs someone to go over all the places we build a Event::HTLCHandlingFailed and make sure it has a unique HTLCDestination with enough info.