Fix fetch-narinfo-files to work in the specific outputs case

guix-data-service/substitutes.scm

@@ -132,9 +132,10 @@
                   build-server-id
                   filtered-narinfos)))))
 
-          (loop (second (last outputs-chunk))
-                total-requested
-                total-narinfos))))))
+          (unless specific-outputs
+            (loop (false-if-exception (second (last outputs-chunk)))
+                  total-requested
+                  total-narinfos)))))))
 
 (define %substitute-query-channel #f)