c/members_manager: validate if node exists in the cluster before update

Previously `members_manager::handle_configuration_update_request` method
did not validate if a node exists in the cluster. This lead to assertion
being triggered when resulting cluster configuration was checked. Added
validation of node existence in the `members_table`.

Fixes: redpanda-data#13108

Signed-off-by: Michal Maslanka <michal@redpanda.com>

src/v/cluster/cluster_utils.cc

@@ -498,11 +498,11 @@ std::optional<ss::sstring> check_result_configuration(
   const members_table::cache_t& current_brokers,
   const model::broker& new_configuration) {
     auto it = current_brokers.find(new_configuration.id());
-    vassert(
-      it != current_brokers.end(),
-      "When broker configuration is being updated the broker must exists. "
-      "Updating broker {} configuration.",
-      new_configuration);
+    if (it == current_brokers.end()) {
+        return fmt::format(
+          "broker {} does not exists in list of cluster members",
+          new_configuration.id());
+    }
     auto& current_configuration = it->second.broker;
     /**
      * do no allow to decrease node core count