Gulp build error to browser

gulpfile.js

//npm i gulp gulp-stylus gulp-plumber browser-sync bs-fullscreen-message

var gulp = require('gulp');
var stylus = require('gulp-stylus');
var plumber = require('gulp-plumber');
var browserSync = require('browser-sync').create();

//Enable "Fullscreen Messages" plugin in Browsersync Plugins web settings (http://localhost:3001/plugins)
//Notification template
var bsError = function(data) {
	browserSync.sockets.emit('fullscreen:message', {
		title: data.plugin + ': Ахтунг! У нас ' +data.name ,
		body: data.message
	}
)};

//Static Server + watching styl files
gulp.task('serve', ['styl'], function() {

	browserSync.init({
		server: "./app",
		plugins: ['bs-fullscreen-message']
	});

	gulp.watch("app/style/*.styl", ['styl']);
});

// Compile styl into CSS & auto-inject into browsers
gulp.task('styl', function() {
	return gulp.src("app/style/*.styl")
		.pipe(plumber(bsError))
		.pipe(stylus())
		.pipe(gulp.dest("app/css"))
		.pipe(browserSync.stream());
});
gulp.task('default', ['serve']);