bug#48561: "Daemon not running" exception when avahi-daemon is not running

[Maxime Devos <maximedevos@telenet.be>]

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Severity: minor

My /var/log/guix-daemon.log.* are full of backtraces like these
(and boring messages like SIGPOLL, spurious SIGPOLL, accepted connections
from pid [..], user [...]).

Backtrace:
In ice-9/boot-9.scm:
  1752:10 10 (with-exception-handler _ _ #:unwind? _ # _)
In unknown file:
           9 (apply-smob/0 #<thunk 7f77d4149f60>)
In ice-9/boot-9.scm:
    724:2  8 (call-with-prompt _ _ #<procedure default-prompt-handle…>)
In ice-9/eval.scm:
    619:8  7 (_ #(#(#<directory (guile-user) 7f77d4143c80>)))
In guix/ui.scm:
  2166:12  6 (run-guix-command _ . _)
In ice-9/boot-9.scm:
  1752:10  5 (with-exception-handler _ _ #:unwind? _ # _)
  1747:15  4 (with-exception-handler #<procedure 7f77d0deeed0 at ic…> …)
In guix/scripts/discover.scm:
    141:8  3 (_)
In guix/avahi.scm:
   171:17  2 (avahi-browse-service-thread _ #:types _ #:ignore-local? …)
In unknown file:
           1 (make-client #<poll 7f77d0de41c0> () #<procedure client…>)
In ice-9/boot-9.scm:
  1685:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1685:16: In procedure raise-exception:
Throw to key `avahi-error' with args `(#<avahi-error-enum Daemon not running> make-client)'.

Some kind of warning message when the daemon is not running makes sense,
but a full backtrace seems unneeded.

Maybe print a line like:

  warning: avahi daemon is not running, cannot auto-discover substitute servers!

instead.

Greetings,
Maxime.

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