[FIX] web: restore proper public widgets onFailure in old guardedCatch

Commit [1] made a mistake when adapting the guardedCatch handler.

[1]: odoo@fcb16a3

closes odoo#163486

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/web/static/src/legacy/js/public/public_root.js

@@ -246,7 +246,9 @@ export const PublicRoot = publicWidget.RootWidget.extend({
         this._startWidgets(ev.data.$target, ev.data.options)
             .then(ev.data.onSuccess)
             .catch((e) => {
-                ev.data.onFailure(e);
+                if (ev.data.onFailure) {
+                    ev.data.onFailure(e);
+                }
                 if (!(e instanceof RPCError)) {
                     return Promise.reject(e);
                 }