bug#34015: guix copy error message is quite difficult to understand

["Clément Lassieur" <clement@lassieur.org>]

Hi,

This is what happens when /etc/profile isn't sourced in the remote
non-interactive shell on guix copy.

I find it difficult to understand.  I think the error message should
lead us to a way to fix the issue.

--8<---------------cut here---------------start------------->8---
sending 1 store item (0 MiB) to '192.168.0.51'...
;;; [2019/01/08 16:48:31.587577, 0] write_to_channel_port: [GSSH ERROR] Remote channel is closed: #<input-output: channel (open) 16bba60>
Backtrace:
          10 (primitive-load "/home/clement/.config/guix/current/bin…")
In guix/ui.scm:
  1644:12  9 (run-guix-command _ . _)
In ice-9/boot-9.scm:
    829:9  8 (catch srfi-34 #<procedure 169c020 at guix/ui.scm:622:…> …)
    829:9  7 (catch system-error #<procedure 169c0c0 at guix/script…> …)
In guix/scripts/copy.scm:
    80:27  6 (send-to-remote-host _ _)
In guix/ssh.scm:
    313:4  5 (send-files #<build-daemon 256.99 1694f00> _ _ # _ # _)
In guix/store.scm:
  1466:12  4 (export-paths #<build-daemon 256.99 1694f00> _ #<input…> …)
  1446:22  3 (export-path #<build-daemon 256.99 1694f00> _ #<input-…> …)
   644:13  2 (process-stderr _ _)
   607:10  1 (dump-port #<input-output: socket 14> #<input-output: …> …)
In unknown file:
           0 (put-bytevector #<input-output: channel (open) 16bba60> …)

ERROR: In procedure put-bytevector:
Throw to key `guile-ssh-error' with args `("write_to_channel_port" "Remote channel is closed" #<input-output: channel (open) 16bba60> #f)'.
--8<---------------cut here---------------end--------------->8---

Cheers,
Clément