challenge: Really exit with non-zero upon hash mismatch.

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.

doc/guix.texi

@@ -5582,14 +5582,19 @@ $ guix challenge @var{package}
 
 @noindent
 where @var{package} is a package specification such as
-@code{guile-2.0} or @code{glibc:debug}.
+@code{guile@@2.0} or @code{glibc:debug}.
 
 The general syntax is:
 
 @example
 guix challenge @var{options} [@var{packages}@dots{}]
 @end example
 
+When a difference is found between the hash of a locally-built item and
+that of a server-provided substitute, or among substitutes provided by
+different servers, the command displays it as in the example above and
+exits with a non-zero return code.
+
 The one option that matters is:
 
 @table @code

guix/scripts/challenge.scm

@@ -233,9 +233,11 @@ Challenge the substitutes for PACKAGE... provided by one or more servers.\n"))
 
            (run-with-store store
              (mlet* %store-monad ((items  (mapm %store-monad
-                                               ensure-store-item files))
+                                                ensure-store-item files))
                                   (issues (discrepancies items urls)))
                (for-each summarize-discrepancy issues)
+               (unless (null? issues)
+                 (exit 1))
                (return (null? issues)))
              #:system system)))))))
 

scripts/guix.in

@@ -64,4 +64,9 @@
       (apply guix-main (command-line))))
 
   (maybe-augment-load-paths!)
+
+  ;; XXX: It would be more convenient to change it to:
+  ;;   (exit (run-guix-main))
+  ;; but since the 'guix' command is not updated by 'guix pull', we cannot
+  ;; really do it now.
   (run-guix-main))