upstream: 'download-tarball' gracefully handles missing signatures.

This avoids a backtrace with "guix refresh -u rdiff-backup", which has
".asc" signatures instead of ".sig".

* guix/upstream.scm (download-tarball): Gracefully handle the case where
SIG is false.
* guix/gnu-maintenance.scm (latest-savannah-release): Add comment about
'file->signature'.

guix/gnu-maintenance.scm

@@ -650,6 +650,9 @@ releases are on gnu.org."
          (directory (dirname (uri-path uri)))
          (rewrite   (url-prefix-rewrite %savannah-base
                                         "mirror://savannah")))
+    ;; Note: We use the default 'file->signature', which adds ".sig", but not
+    ;; all projects on Savannah follow that convention: some use ".asc" and
+    ;; perhaps some lack signatures altogether.
     (and=> (latest-html-release package
                                 #:base-url %savannah-base
                                 #:directory directory)

guix/upstream.scm

@@ -326,10 +326,17 @@ values: 'interactive' (default), 'always', and 'never'."
                                (built-derivations (list drv))
                                (return (derivation->output-path drv))))))))
           (let-values (((status data)
-                        (gnupg-verify* sig data #:key-download key-download)))
+                        (if sig
+                            (gnupg-verify* sig data
+                                           #:key-download key-download)
+                            (values 'missing-signature data))))
             (match status
               ('valid-signature
                tarball)
+              ('missing-signature
+               (warning (G_ "failed to download detached signature from ~a~%")
+                        signature-url)
+               #f)
               ('invalid-signature
                (warning (G_ "signature verification failed for '~a' (key: ~a)~%")
                         url data)