publish: Gracefully handle the lack of a deriver.

* guix/scripts/publish.scm (narinfo-string): Catch 'system-error' around
  'load-derivation' call; return BASE-INFO upon ENOENT.  This allows us
  to return the narinfo even if DERIVER is missing.  Before that, the
  exception would be uncaught, leading to 500 Internal Error on the
  client side.

guix/scripts/publish.scm

@@ -164,10 +164,19 @@ References: ~a~%"
          ;; info for a derivation.
          (info       (if (string-null? deriver)
                          base-info
+                         (catch 'system-error
+                           (lambda ()
                              (let ((drv (load-derivation deriver)))
                                (format #f "~aSystem: ~a~%Deriver: ~a~%"
                                        base-info (derivation-system drv)
-                             (basename deriver)))))
+                                       (basename deriver))))
+                           (lambda args
+                             ;; DERIVER might be missing, but that's fine:
+                             ;; it's only used for <substitutable> where it's
+                             ;; optional.  'System' is currently unused.
+                             (if (= ENOENT (system-error-errno args))
+                                 base-info
+                                 (apply throw args))))))
          (signature  (base64-encode-string
                       (canonical-sexp->string (signed-string info)))))
     (format #f "~aSignature: 1;~a;~a~%" info (gethostname) signature)))

guix/scripts/substitute.scm

@@ -703,7 +703,7 @@ Internal tool to substitute a pre-built binary to a local build.\n"))
 ;;;
 
 (define (display-narinfo-data narinfo)
-  "Write to the current output port the contents of NARINFO is the format
+  "Write to the current output port the contents of NARINFO in the format
 expected by the daemon."
   (format #t "~a\n~a\n~a\n"
           (narinfo-path narinfo)