ssh: Improve error reporting when retrieving files.

'guix copy --from' now reports messages much more useful than "failed to
retrieve files".

* guix/ssh.scm (store-export-channel)[export]: Wrap 'use-modules' in
'catch' and 'with-store' in 'guard'.  Check for invalid items.  Write a
status sexp on stdout.
(raise-error): New macro.
(retrieve-files): Read the initial status sexp and report errors
accordingly.