marionette: Make it easier to debug REPL read failures.

Log the remaining contnet written to the REPL, so that there's more to go on
than:

  socket:5:14: Unknown # object: "#<"

* gnu/build/marionette.scm (marionette-eval): Catch exceptions from read and
log the remainder of the content from the REPL.

gnu/build/marionette.scm

@@ -178,7 +178,18 @@ QEMU monitor and to the guest's backdoor REPL."
     (($ <marionette> command pid monitor (= force repl))
      (write exp repl)
      (newline repl)
-     (read repl))))
+     (with-exception-handler
+         (lambda (exn)
+           (simple-format
+            (current-error-port)
+            "error reading marionette response: ~A
+  remaining response: ~A\n"
+            exn
+            (get-line repl))
+           (raise-exception exn))
+       (lambda ()
+         (read repl))
+       #:unwind? #t))))
 
 (define* (wait-for-file file marionette
                         #:key (timeout 10) (read 'read))