How to make validation in a remote event receiver - ItemAdding

I have a remote event receiver on the item adding event of a list called clients. Everytime a new list item is created a new subsite is created.

However I want to make a validation, if site already exists (exception thrown by method createweb), then a message should be shown and form should stay so the user can correct the fields, however the page is redirected to AllItems.aspx

As you can see I am using SPRemoteEventResult with status and error message, but this seems to be useless, error is never shown anywhere.

 public SPRemoteEventResult ProcessEvent(SPRemoteEventProperties properties)
        {
            SPRemoteEventResult result = new SPRemoteEventResult();
            try
            {              
                switch (properties.EventType)
                {
                    case SPRemoteEventType.AppInstalled:
                        AppEvents.HandleAppInstalled(properties);
                        result.Status = SPRemoteEventServiceStatus.Continue;
                        break;
                    case SPRemoteEventType.AppUninstalling:
                        AppEvents.HandleAppUninstalled(properties);
                        result.Status = SPRemoteEventServiceStatus.Continue;
                        break;
                    case SPRemoteEventType.AppUpgraded:
                        AppEvents.HandleAppUpgraded(properties);
                        result.Status = SPRemoteEventServiceStatus.Continue;
                        break;
                    case SPRemoteEventType.ItemAdding:
                        if(properties.ItemEventProperties.ListTitle=="Clientes")
                        {
                            return Clients.CreateClientSiteAndUpdateSiteOnList(properties);                           
                        }
                        if (properties.ItemEventProperties.ListTitle == "Proyectos")
                        {
                            return Projects.CreateProjectSiteAndUpdateSiteOnList(properties);
                        }
                        break;  

                }
                return result;
            }
            catch (Exception ex)
            {
                //Log message
                result.ErrorMessage = "Capatech.Intranet:  " + ex.Message;
                result.Status = SPRemoteEventServiceStatus.CancelWithError;
                return result;
            }
        }



 public static SPRemoteEventResult CreateClientSiteAndUpdateSiteOnList(SPRemoteEventProperties properties)
        {
            SPRemoteEventResult result = new SPRemoteEventResult();
            try
            {
                using (ClientContext clientContext = TokenHelper.CreateRemoteEventReceiverClientContext(properties))
                {
                    if (clientContext != null)
                    {
                        var afterProperties = properties.ItemEventProperties.AfterProperties;
                        var beforeProperties = properties.ItemEventProperties.BeforeProperties;

                        string nombrecliente = afterProperties["Nombre_x0020_Cliente"].ToString();
                        int indexOfhash = nombrecliente.IndexOf("#");
                        int indexOfPipe = nombrecliente.IndexOf("|");
                        string clientName = nombrecliente.Substring(indexOfhash + 1, indexOfPipe - indexOfhash -1);

                        string site_title = afterProperties["NIT"].ToString() + " - " + clientName;
                        string site_url = afterProperties["NIT"].ToString();



                        //get the subweb clients which is the host of all client sites
                        clientContext.Load(clientContext.Web, w => w.Webs);
                        clientContext.ExecuteQuery();
                        var subWebClients = (from w in clientContext.Web.Webs where w.Title == "clientes" select w).SingleOrDefault();

                        Web newWeb = subWebClients.CreateWeb(site_title, site_url, site_title, "STS#0", 1033);
                        clientContext.Load(newWeb);
                        clientContext.ExecuteQuery();


                        result.ChangedItemProperties.Add("Sitio_x0020_Cliente", newWeb.Url + "," + site_title + " Sitio");
                        result.Status = SPRemoteEventServiceStatus.Continue;
                        return  result;
                    }
                    else
                    {
                        result.ErrorMessage = "No se pudo encontrar el contexto";
                        result.Status=SPRemoteEventServiceStatus.CancelWithError;
                        return result;
                    }
                }  
            }
            catch (Exception ex)
            {
                result.ErrorMessage = ex.Message;
                result.Status = SPRemoteEventServiceStatus.CancelWithError;
                return result;
            }                    
        }