refresh: Distinguish between "no updater" and "failing updater".

Previously, something like "guix refresh texmacs" would report "no
updater".  Now, it reports that the 'gnu-ftp' updater failed to list
releases.

* guix/upstream.scm (lookup-updater): Use 'find' instead of 'any' to
return the <upstream-updater>.
(package-latest-release): Adjust accordingly.
* guix/scripts/refresh.scm (check-for-package-update): When
'package-latest-release' returns #f, distinguish between "no updater"
and "failing updater".

guix/scripts/refresh.scm

@@ -368,8 +368,16 @@ the latest known version of ~a (~a)~%")
                      (upstream-source-version source)))))))
     (#f
      (when warn?
-       (warn-no-updater package)))))
-
+       ;; Distinguish between "no updater" and "failing updater."
+       (match (lookup-updater package updaters)
+         ((? upstream-updater? updater)
+          (warning (package-location package)
+                   (G_ "'~a' updater failed to determine available \
+releases for ~a~%")
+                   (upstream-updater-name updater)
+                   (package-name package)))
+         (#f
+          (warn-no-updater package)))))))
 
 
 ;;;

guix/upstream.scm

@@ -245,18 +245,18 @@ correspond to the same version."
 (define (lookup-updater package updaters)
   "Return an updater among UPDATERS that matches PACKAGE, or #f if none of
 them matches."
-  (any (match-lambda
-         (($ <upstream-updater> name description pred latest)
-          (and (pred package) latest)))
-       updaters))
+  (find (match-lambda
+          (($ <upstream-updater> name description pred latest)
+           (pred package)))
+        updaters))
 
 (define (package-latest-release package updaters)
   "Return an upstream source to update PACKAGE, a <package> object, or #f if
 none of UPDATERS matches PACKAGE.  It is the caller's responsibility to ensure
 that the returned source is newer than the current one."
   (match (lookup-updater package updaters)
-    ((? procedure? latest-release)
-     (latest-release package))
+    ((? upstream-updater? updater)
+     ((upstream-updater-latest updater) package))
     (_ #f)))
 
 (define (package-latest-release* package updaters)