Pass daemon errors back to RPC handler

[niftynei]

Instead of “OPENINGD died”, show actual error message that openingd generates.

Right now you can find them in the logs, but it'd be a better UX if we passed through the error reason to the RPC handler in lightningd.

[shinghim]

Couldn't find "OPENINGD died" in the code. Makes me think that this has already been resolved

[NishantBansal2003]

Hi @niftynei, is this issue available to work on?

[niftynei]

Hm I'm not sure that it's still relevant, it's been a while since I've looked at what error codes openingd returns.