offload: Gracefully report connection failures.

* guix/scripts/offload.scm (open-ssh-session): Check the return value of
'connect!'.  Call 'leave' when it's not 'ok.

guix/scripts/offload.scm

@@ -177,8 +177,8 @@ private key from '~a': ~a")
                                ;; exchanging full archives.
                                #:compression "zlib"
                                #:compression-level 3)))
-    (connect! session)
-
+    (match (connect! session)
+      ('ok
        ;; Authenticate the server.  XXX: Guile-SSH 0.10.1 doesn't know about
        ;; ed25519 keys and 'get-key-type' returns #f in that case.
        (let-values (((server)   (get-server-public-key session))
@@ -201,7 +201,11 @@ instead of '~a' of type '~a'~%")
            (leave (_ "SSH public key authentication failed for '~a': ~a~%")
                   (build-machine-name machine) (get-error session))))
 
-    session))
+       session)
+      (x
+       ;; Connection failed or timeout expired.
+       (leave (_ "failed to connect to '~a': ~a~%")
+              (build-machine-name machine) (get-error session))))))
 
 (define* (connect-to-remote-daemon session
                                    #:optional