The goal is to make it easier to diagnose substitute
misconfiguration (where we’re passing a substitute URL whose
corresponding key is not authorized).
Suggested by Emmanuel Agullo.
* guix/scripts/weather.scm (check-narinfo-authorization): New procedure.
(report-server-coverage): Use it.
* doc/guix.texi (Invoking guix weather): Document it.
(Getting Substitutes from Other Servers): Add “Troubleshooting” frame.
Change-Id: I0a049c39eefb10d6a06634c8b16aa86902769791