title: webpack exits without any error codes if a module is missing
body: <p>I see the error message - it's red - but i'd like it to return a non-zero exit code so i know it errored out. what configuration am i missing? I am not using the no errors plugin</p>
  
  <pre><code>'use strict'
  
  const webpack = require('webpack')
  const path = require('path')
  
  const babel_cache_path = require('../config/babel').cache_path
  const port = require('../config').port
  
  module.exports = {
    entry: {
      index: './client/index',
      dev: [
        'webpack-dev-server/client?http://localhost:' + (port + 1),
        'webpack/hot/only-dev-server',
      ],
    },
    output: {
      path: path.resolve('build'),
      publicPath: 'http://localhost:' + (port + 1) + '/',
      filename: '[name].js',
      chunkFilename: '[id].[name].js',
    },
    plugins: [
      new webpack.IgnorePlugin(/^\.\/locale$/, /moment$/),
      new webpack.DefinePlugin({
        'process.env.NODE_ENV': JSON.stringify(process.env.NODE_ENV || 'development'),
      }),
      new webpack.HotModuleReplacementPlugin(),
      // new webpack.NoErrorsPlugin(),
    ],
    resolve: {
      extensions: [
        '',
        '.js',
        '.jsx'
      ]
    },
    module: {
      loaders: [
        // babel + jsx loader
        {
          test: /\.jsx?$/,
          exclude: /node_modules/,
          loaders: [
            `babel?cacheDirectory=${babel_cache_path}`
          ]
        },
        // json loader
        {
          test: /\.json$/,
          loaders: [
            'json'
          ]
        },
        // css loader
        {
          test: /\.css$/,
          loaders: [
            'style',
            'css?modules&amp;sourceMap&amp;importLoaders=1&amp;localIdentName=[path][name]---[local]---[hash:base64:5]',
            'postcss'
          ]
        }
      ],
    },
    postcss () {
      return require('postcss-jongleberry')({
        minify: false
      })
    },
    // no node builtins
    node: false
  }
  </code></pre>