shepherd: hardening error handling

[Attila Lendvai <attila@lendvai.name>]

dear Guix,

i'm working on hardening shepherd's error handling and logging to debug an issue that i'm facing. these changes escalated quickly, so i'm writing to clarify a few things before i shape the codebase into a direction that the maintainers will not accept.

the codebase seems to use catch/throw, and at some places with comments like "for Guile 2.2". what is the minimum guile version that the shepherd codebase wants to support? the README says "GNU Guile 3.0.x or 2.2.x". is this still intended? or can i assume guile 3? i.e. use with-exception-handler, raise-exception, guard, &co. instead of catch/throw with key and args?

some WIP commits are available at:

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

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• attila lendvai
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“It is only when the people become ignorant and corrupt, when they degenerate into a populace, that they are incapable of exercising the sovereignty. Usurpation is then an easy attainment, and an usurper soon found. The people themselves become the willing instruments of their own debasement and ruin.”
	— James Monroe (1758–1831), 5th president of the USA