[shepherd] several patches that i deem ready

[Attila Lendvai <attila@lendvai.name>]

dear Guix, Ludo,

i have prepared the rest of my commits that were needed to hunt down the shepherd hanging bug. you can find them at:

https://codeberg.org/attila-lendvai-patches/shepherd/commits/branch/attila

there's some dependency among the commits, so sending them to debbugs would be either as one big series of commits, or a hopeless labirinth of patches otherwise.

therefore i recommend the following workflow instead (assuming that Ludo is pretty much the only one hacking on shepherd):

Ludo, please take a look at my branch, and cherry-pick whatever you are happy with. then based on your feedback, and the new main branch, i'll rebase and refine my commits and give you a head's up when it's ready for another merge/review.

the commits are more or less ordered in least controversial order, modulo dependencies.

the main additions are:

- a multi-layered error handler that got employed at various points in
  the codebase. this makes shepherd much more resilient, even in case
  of nested errors, and much more communicative in the log when errors
  end up happening.

- a lightweight logging infrastructure together with plenty of log
  lines throughout the codebase, and some hints in the README on how
  to turn log lines gray in emacs (i.e. easily ignorable).

looking forward to your feedback,

-- 
• attila lendvai
• PGP: 963F 5D5F 45C7 DFCD 0A39
--
“What you do speaks so loud I cannot hear what you say.”
	— Ralph Waldo Emerson (1803–1882)