bug#63972: specifying a substitute server without adding its PGP key silently ignores it

[Attila Lendvai]

i've installed a new guix, and at the first `guix system reconfigure` i 
specified a substitute server using --substitute-urls for That Other Channel. i 
had to do this, because the config.scm that contains the substitute 
specification is yet to be applied.

it didn't work. it prints everything as usual, including the 100% message for 
that substitute server, but it starts to build packages locally for which 
substitutes are available. i haven't noticed any indication that there's a 
problem with any of the substitute servers.

once i've downloaded the .pub and i finally did the right incantation (sudo 
guix archive --authorize < signing-key.pub), then it started to download the 
substitutes as i expected.

i would much prefer a behavior where a "cryptyc" exception and backtrace is 
printed by a toplevel error handler. it has cost me about an hour of my life.

i'd suggest the following general strategy for the entire codebase in general:

throw exceptions, and let them fly all the way up to the toplevel error handler 
that should print it with a backtrace. this should be the baseline, and only 
then start adding very specific exception handlers to print friendly and 
localizable error messages for various situations, and only ever swallow 
exceptions when it's really justified. e.g. a file-not-found error in an 
ensure-file-deleted function.

-- 
• attila lendvai
• PGP: 963F 5D5F 45C7 DFCD 0A39
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