challenge: Use exit code 2 when discrepancies are found.

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.

doc/guix.texi

@@ -5609,7 +5609,8 @@ guix challenge @var{options} [@var{packages}@dots{}]
 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.
+its exit code is 2 (other non-zero exit codes denote other kinds of
+errors.)
 
 The one option that matters is:
 

guix/scripts/challenge.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2015 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2015, 2016 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -237,7 +237,7 @@ Challenge the substitutes for PACKAGE... provided by one or more servers.\n"))
                                   (issues (discrepancies items urls)))
                (for-each summarize-discrepancy issues)
                (unless (null? issues)
-                 (exit 1))
+                 (exit 2))
                (return (null? issues)))
              #:system system)))))))