offload: Warn about SSH client issues.

Suggested by Ricardo Wurmus <ricardo.wurmus@mdc-berlin.de>.

* guix/scripts/offload.scm (remote-pipe): Remove unneeded 'catch'.
  (machine-load): Check the exit value  upon (close-pipe pipe).  Call
  'warning' when it is non-zero.

guix/scripts/offload.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2015 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -191,8 +191,6 @@ not be started."
       (lambda ()
         (write str))))
 
-  (catch 'system-error
-    (lambda ()
   ;; Let the child inherit ERROR-PORT.
   (with-error-to-port error-port
     (apply open-pipe* mode %lshg-command
@@ -206,10 +204,6 @@ not be started."
            (if quote?
                (map shell-quote command)
                command))))
-    (lambda args
-      (warning (_ "failed to execute '~a': ~a~%")
-               %lshg-command (strerror (system-error-errno args)))
-      #f)))
 
 
 ;;;
@@ -534,8 +528,13 @@ success, #f otherwise."
   "Return the load of MACHINE, divided by the number of parallel builds
 allowed on MACHINE."
   (let* ((pipe   (remote-pipe machine OPEN_READ `("cat" "/proc/loadavg")))
-         (line (read-line pipe)))
-    (close-pipe pipe)
+         (line   (read-line pipe))
+         (status (close-pipe pipe)))
+    (unless (eqv? 0 (status:exit-val status))
+      (warning (_ "failed to obtain load of '~a': SSH client exited with ~a~%")
+               (build-machine-name machine)
+               (status:exit-val status)))
+
     (if (eof-object? line)
         +inf.0    ;MACHINE does not respond, so assume it is infinitely loaded
         (match (string-tokenize line)