Suggested by John Darrington <john@darrington.wattle.id.au>.
* guix/scripts/challenge.scm (guix-challenge): Exit with 2 when MISSING
is not empty.
* doc/guix.texi (Invoking guix challenge): Document it.
Reported by John Darrington.
* guix/scripts/challenge.scm (guix-challenge): Add an explicit 'exit'
call when ISSUES is empty.
* scripts/guix.in: Add comment about 'exit'.
* doc/guix.texi (Invoking guix challenge): Mention the behavior and exit
code.
* guix/scripts/substitute.scm (lookup-narinfos): Filter out #f values
from CACHED, such that the end result is exactly a list of narinfos,
not interspersed with #f.
* guix/scripts/challenge.scm (discrepancies): Assume REMOTE is a list of
narinfos.
Ludovic Courtès