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'.