Show the répartition validation error message instead of failing silently

Editing a ligne_comptable with an inconsistent répartition correctly
blocked the save (setErrorByName() on field_repartition), but the
message text had nowhere to render: it normally goes through the page's
status-messages region, which isn't part of this standalone AJAX
ReplaceCommand fragment. The form re-render did mark every répartition
row with a subtle red outline, but with no visible explanation the modal
just looked stuck.

figli_compta_ledger_node_form_ajax_submit() now also emits a
MessageCommand for each form error, mirroring the pattern already used
for the success path's messenger() messages. Verified live: breaking a
répartition sum and clicking Save now shows a proper error toast with
the exact mismatch amount, the modal correctly stays open, and the node
is confirmed unchanged in the database.

web/modules/custom/figli_compta_ledger/figli_compta_ledger.module

@@ -122,6 +122,16 @@ function figli_compta_ledger_validate_repartition(array &$form, FormStateInterfa
 function figli_compta_ledger_node_form_ajax_submit(array $form, FormStateInterface $form_state) {
   $response = new AjaxResponse();
   if ($form_state->getErrors()) {
+    // The re-rendered form below does mark every répartition row with an
+    // `error`/`has-error` CSS class (a subtle red outline), but
+    // setErrorByName()'s message text itself has nowhere to render --
+    // that normally goes through the page's status-messages region,
+    // which isn't part of this standalone AJAX-replaced form fragment.
+    // Without this, the message was silently discarded: the modal just
+    // stayed open with reddened fields and no visible explanation.
+    foreach ($form_state->getErrors() as $error) {
+      $response->addCommand(new MessageCommand($error, NULL, ['type' => 'error'], FALSE));
+    }
     unset($form['#prefix'], $form['#suffix']);
     $response->addCommand(new ReplaceCommand('#' . $form['#id'], $form));
     return $response;