gulp error swallowing with plumber

gulpfile.js

const gulp         = require('gulp'),
      sass         = require('gulp-sass'),
      sourcemaps   = require('gulp-sourcemaps'),
      autoprefixer = require('gulp-autoprefixer'),
      cssnano      = require('gulp-cssnano'),
      newer        = require('gulp-newer'),
      concat       = require('gulp-concat'),
      plumber      = require('gulp-plumber'),
      notify       = require('gulp-notify'),
      bs           = require('browser-sync');

function plumbError() {
  return plumber({
    errorHandler: function(err) {
      notify.onError({
        templateOptions: {
          date: new Date()
        },
        title: "Gulp error in " + err.plugin,
        message:  err.formatted
      })(err);

      this.emit('end');
    }
  })
}

/** Sass file handling */
gulp.task('sass', function() {
    return gulp.src("src/front/scss/*.scss")
        .pipe(sourcemaps.init())
        .pipe(plumbError())
        .pipe(sass())
        .pipe(autoprefixer({
          browsers: ['last 2 versions'],
          cascade: false }))
        .pipe(cssnano())
        .pipe(sourcemaps.write('/'))
        .pipe(gulp.dest("src/static/styles"))
        .pipe(bs.stream());
});