ui: Gracefully report failures to connect to the daemon.

* guix/store.scm (&nix-connection-error): New condition type.
  (open-connection): Translate `system-error' during the `connect' call
  into `&nix-connection-error'.
* guix/ui.scm (call-with-error-handling): Add case for `nix-connection-error?'.
* guix/scripts/package.scm (guix-package): Move `open-connection' call
  within `with-error-handling'.
* guix/scripts/pull.scm (guix-pull): Likewise.
* guix/scripts/download.scm (guix-download): Move body within
  `with-error-handling'.

guix/scripts/download.scm

@@ -110,6 +110,7 @@ and the hash of its contents.\n"))
                  (alist-cons 'argument arg result))
                %default-options))
 
+  (with-error-handling
     (let* ((opts  (parse-options))
            (store (open-connection))
            (arg   (assq-ref opts 'argument))
@@ -132,4 +133,4 @@ and the hash of its contents.\n"))
                     (compose sha256 get-bytevector-all)))
            (fmt   (assq-ref opts 'format)))
       (format #t "~a~%~a~%" path (fmt hash))
-    #t))
+      #t)))

guix/scripts/package.scm

@@ -712,8 +712,8 @@ Install, remove, or upgrade PACKAGES in a single transaction.\n"))
 
   (let ((opts (parse-options)))
     (or (process-query opts)
-        (parameterize ((%store (open-connection)))
         (with-error-handling
+          (parameterize ((%store (open-connection)))
             (parameterize ((%guile-for-build
                             (package-derivation (%store)
                                                 (if (assoc-ref opts 'bootstrap?)

guix/scripts/pull.scm

@@ -194,9 +194,9 @@ Download and deploy the latest version of Guix.\n"))
                  (leave (_ "~A: unexpected argument~%") arg))
                %default-options))
 
+  (with-error-handling
     (let ((opts  (parse-options))
           (store (open-connection)))
-    (with-error-handling
       (let ((tarball (download-and-store store)))
         (unless tarball
           (leave (_ "failed to download up-to-date source, exiting\n")))

guix/store.scm

@@ -39,6 +39,9 @@
             nix-server-socket
 
             &nix-error nix-error?
+            &nix-connection-error nix-connection-error?
+            nix-connection-error-file
+            nix-connection-error-code
             &nix-protocol-error nix-protocol-error?
             nix-protocol-error-message
             nix-protocol-error-status
@@ -373,6 +376,11 @@
 (define-condition-type &nix-error &error
   nix-error?)
 
+(define-condition-type &nix-connection-error &nix-error
+  nix-connection-error?
+  (file   nix-connection-error-file)
+  (errno  nix-connection-error-code))
+
 (define-condition-type &nix-protocol-error &nix-error
   nix-protocol-error?
   (message nix-protocol-error-message)
@@ -392,7 +400,15 @@ operate, should the disk become full.  Return a server object."
     ;; Enlarge the receive buffer.
     (setsockopt s SOL_SOCKET SO_RCVBUF (* 12 1024))
 
-    (connect s a)
+    (catch 'system-error
+      (cut connect s a)
+      (lambda args
+        ;; Translate the error to something user-friendly.
+        (let ((errno (system-error-errno args)))
+          (raise (condition (&nix-connection-error
+                             (file file)
+                             (errno errno)))))))
+
     (write-int %worker-magic-1 s)
     (let ((r (read-int s)))
       (and (eqv? r %worker-magic-2)

guix/ui.scm

@@ -111,6 +111,10 @@ General help using GNU software: <http://www.gnu.org/gethelp/>"))
                (leave (_ "~a:~a:~a: error: package `~a' has an invalid input: ~s~%")
                       file line column
                       (package-full-name package) input)))
+            ((nix-connection-error? c)
+             (leave (_ "error: failed to connect to `~a': ~a~%")
+                    (nix-connection-error-file c)
+                    (strerror (nix-connection-error-code c))))
             ((nix-protocol-error? c)
              ;; FIXME: Server-provided error messages aren't i18n'd.
              (leave (_ "error: build failed: ~a~%")