import: hackage: Fix invalid use of 'leave'.

Reported by Paul van der Walt <paul@denknerd.org>
in <http://bugs.gnu.org/21829>.

* guix/scripts/import/hackage.scm (guix-import-hackage): Add missing
  argument in call to 'leave'.

guix/scripts/import/hackage.scm

@@ -134,9 +134,9 @@ from standard input~%")))))
           ((package-name)
            (run-importer package-name opts
                          (lambda ()
-                           (leave
-                            (_ "failed to download cabal file for package '~a'~%"))
-                           package-name)))
+                           (leave (_ "failed to download cabal file \
+for package '~a'~%")
+                                  package-name))))
           (()
            (leave (_ "too few arguments~%")))
           ((many ...)