bug#35300: 27.0.50; emacsclient on remote sessions no longer works

[Óscar Fuentes]

retitle Improve emacsclient error message to inform about new requirements
stop

Glenn Morris <rgm@gnu.org> writes:

> Works for me. I guess your ssh is not setting XDG_RUNTIME_DIR.
> This is handled by pam_systemd, ref eg
> https://www.freedesktop.org/software/systemd/man/pam_systemd.html

I have

UsePAM no

in my sshd config. Thanks Glenn.

> See discussion in https://debbugs.gnu.org/33847

Before reporting this issue I found and skimmed over that one. What a
mess.

My main concern about this is to ensure that any change that breaks
functionality goes with prominent notes about the change and how to
adapt to it. Showing just the standard error message in the output of
emacsclient when it knows the probable origin of the problem is not user
friendly. We must give a proper note about the existence of new modes of
failure and explain (or at least hint) how to deal with them.