bug#31901: Incorrect make-network-process + nowait state handling for non-existing unix sockets in emacs-26.1

[Lars Ingebrigtsen]

OK, I've now fixed the async error case (on master), I think:

  (process-live-p socket)
  nil
  (process-status socket)
  file-missing

I've fixed this on master, because I'm not 100% sure that my fix is
completely correct.  Should we perhaps also call a sentinel in the async
error case?

But we should, in any case, perhaps cherry-pick this fix into Emacs
26.2.

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