offload: 'test' reports Guile and module errors more nicely.

Fixes <https://bugs.gnu.org/26008>.
Reported by Myles English <mylesenglish@gmail.com>.

* guix/ssh.scm (retrieve-files*): Move error reporting to...
(report-guile-error, report-module-error): ... here.  New procedures.
* guix/scripts/offload.scm (assert-node-repl): Use 'report-guile-error'.
(assert-node-has-guix): Explicitly check for 'use-modules' first.  Use
'report-module-error'.

guix/scripts/offload.scm

@@ -542,8 +542,7 @@ slot (which must later be released with 'release-build-slot'), or #f and #f."
   "Bail out if NODE is not running Guile."
   (match (node-guile-version node)
     (#f
-     (leave (G_ "Guile could not be started on '~a'~%")
-            name))
+     (report-guile-error name))
     ((? string? version)
      ;; Note: The version string already contains the word "Guile".
      (info (G_ "'~a' is running ~a~%")
@@ -551,6 +550,17 @@ slot (which must later be released with 'release-build-slot'), or #f and #f."
 
 (define (assert-node-has-guix node name)
   "Bail out if NODE lacks the (guix) module, or if its daemon is not running."
+  (catch 'node-repl-error
+    (lambda ()
+      (match (node-eval node
+                        '(begin
+                           (use-modules (guix))
+                           (and add-text-to-store 'alright)))
+        ('alright #t)
+        (_ (report-module-error name))))
+    (lambda (key . args)
+      (report-module-error name)))
+
   (catch 'node-repl-error
     (lambda ()
       (match (node-eval node
@@ -563,7 +573,7 @@ slot (which must later be released with 'release-build-slot'), or #f and #f."
          (info (G_ "Guix is usable on '~a' (test returned ~s)~%")
                name str))
         (x
-         (leave (G_ "failed to use Guix module on '~a' (test returned ~s)~%")
+         (leave (G_ "failed to talk to guix-daemon on '~a' (test returned ~s)~%")
                 name x))))
     (lambda (key . args)
       (leave (G_ "remove evaluation on '~a' failed:~{ ~s~}~%")

guix/ssh.scm

@@ -41,7 +41,10 @@
             send-files
             retrieve-files
             retrieve-files*
-            remote-store-host))
+            remote-store-host
+
+            report-guile-error
+            report-module-error))
 
 ;;; Commentary:
 ;;;
@@ -365,21 +368,9 @@ from REMOTE.  When RECURSIVE? is true, retrieve the closure of FILES."
          (lambda ()
            (close-port port))))
       ((? eof-object?)
-       (raise-error (G_ "failed to start Guile on remote host '~A': exit code ~A")
-                    (remote-store-host remote)
-                    (channel-get-exit-status port)
-                    (=> (G_ "Make sure @command{guile} can be found in
-@code{$PATH} on the remote host.  Run @command{ssh ~A guile --version} to
-check.")
-                        (remote-store-host remote))))
+       (report-guile-error (remote-store-host remote)))
       (('module-error . _)
-       ;; TRANSLATORS: Leave "Guile" untranslated.
-       (raise-error (G_ "Guile modules not found on remote host '~A'")
-                    (remote-store-host remote)
-                    (=> (G_ "Make sure @code{GUILE_LOAD_PATH} includes Guix'
-own module directory.  Run @command{ssh ~A env | grep GUILE_LOAD_PATH} to
-check.")
-                        (remote-store-host remote))))
+       (report-module-error (remote-store-host remote)))
       (('connection-error file code . _)
        (raise-error (G_ "failed to connect to '~A' on remote host '~A': ~a")
                     file (remote-store-host remote) (strerror code)))
@@ -406,4 +397,25 @@ LOCAL.  When RECURSIVE? is true, retrieve the closure of FILES."
                    #:import (lambda (port)
                               (import-paths local port))))
 
+
+;;;
+;;; Error reporting.
+;;;
+
+(define (report-guile-error host)
+  (raise-error (G_ "failed to start Guile on remote host '~A'") host
+               (=> (G_ "Make sure @command{guile} can be found in
+@code{$PATH} on the remote host.  Run @command{ssh ~A guile --version} to
+check.")
+                   host)))
+
+(define (report-module-error host)
+  "Report an error about missing Guix modules on HOST."
+  ;; TRANSLATORS: Leave "Guile" untranslated.
+  (raise-error (G_ "Guile modules not found on remote host '~A'") host
+               (=> (G_ "Make sure @code{GUILE_LOAD_PATH} includes Guix'
+own module directory.  Run @command{ssh ~A env | grep GUILE_LOAD_PATH} to
+check.")
+                   host)))
+
 ;;; ssh.scm ends here