private void handle400(RoutingContext context, String message) {
                if (message != null) context.response().setStatusMessage(message);
                context.fail(400);
            }
@author Marc Savy {@literal <marc@rhymewithgravy.com>}
gateway/platforms/vertx3/vertx3/src/main/java/io/apiman/gateway/platforms/vertx3/api/auth/KeycloakOAuthFactory.java