offload: Do not abort when a machine is unreachable.

* guix/scripts/offload.scm (machine-load): Wrap 'open-ssh-session' call
in 'false-if-exception'; return +inf.0 if it returns #f.

guix/scripts/offload.scm

@@ -493,10 +493,11 @@ be read."
 
 (define (machine-load machine)
   "Return the load of MACHINE, divided by the number of parallel builds
-allowed on MACHINE."
+allowed on MACHINE.  Return +∞ if MACHINE is unreachable."
   ;; Note: This procedure is costly since it creates a new SSH session.
-  (let* ((session (open-ssh-session machine))
-         (pipe    (open-remote-pipe* session OPEN_READ
+  (match (false-if-exception (open-ssh-session machine))
+    ((? session? session)
+     (let* ((pipe (open-remote-pipe* session OPEN_READ
                                      "cat" "/proc/loadavg"))
             (line (read-line pipe)))
        (close-port pipe)
@@ -514,6 +515,8 @@ allowed on MACHINE."
                 normalized))
              (_
               +inf.0)))))        ;something's fishy about MACHINE, so avoid it
+    (_
+     +inf.0)))                      ;failed to connect to MACHINE, so avoid it
 
 (define (machine-lock-file machine hint)
   "Return the name of MACHINE's lock file for HINT."