fix: Store the exception and not an array with the exception for unca…

…ught promise exceptions (#20840) (#20841)

Co-authored-by: Artur <[email protected]>

flow-server/src/main/java/com/vaadin/flow/server/communication/IndexHtmlRequestHandler.java

@@ -287,7 +287,7 @@ private void catchErrorsInDevMode(Document indexDocument) {
                 + //
                 "};" + //
                 "window.addEventListener('unhandledrejection', e => {" + //
-                "    window.Vaadin.ConsoleErrors.push([e.reason]);" + //
+                "    window.Vaadin.ConsoleErrors.push(e.reason);" + //
                 "});" //
         );
     }