title: webpack-dev-server return "configuration has an unknown property 'error'"
body: <p>I have webpack^3.0.0 and webpack-dev-server^2.9.1 installed (both on the project and globally).
  Upon running <code>webpack-dev-server</code> in the project root, I am presented with the error</p>
  
  <pre><code>Invalid configuration object. webpack-dev-server has been initialised 
  using a configuration object that does not match the API schema.
   - configuration has an unknown property 'error'. These properties are 
  valid:
  object { hot?, hotOnly?, lazy?, bonjour?, host?, allowedHosts?, 
  filename?, publicPath?, port?, socket?, watchOptions?, headers?, 
  clientLogLevel?, overlay?, progress?, key?, cert?, ca?, pfx?, 
  pfxPassphrase?, requestCert?, inline?, disableHostCheck?, public?, 
  https?, contentBase?, watchContentBase?, open?, useLocalIp?, openPage?, 
  features?, compress?, proxy?, historyApiFallback?, staticOptions?, 
  setup?, before?, after?, stats?, reporter?, noInfo?, quiet?, 
  serverSideRender?, index?, log?, warn? }
  </code></pre>
  
  <p>the webpack.config.js file at the projet root contains the following:</p>
  
  <pre><code>module.exports = {
    entry: {
      app: [
        './src/index.js'
      ]
    },
    module: {
      loaders: [{
        test: /\.jsx?$/,
        exclude: /node_modules/,
        loader: 'babel-loader'
      }]
    },
    resolve: {
      extensions: ['*', '.js', '.jsx']
    },
    output: {
      path: __dirname + '/dist',
      publicPath: '/',
      filename: 'bundle.js'
    },
    devServer: {
      inline: false,
      contentBase: './dist'
    }
  };
  </code></pre>
  
  <p>I have tried following the advice in <a href="https://stackoverflow.com/questions/42060243/invalid-configuration-object-webpack-has-been-initialised-using-a-configuration">this SO question</a> but to no avail. How can I get webpack to start?</p>
  
  <p>I've also noticed that when I installed webpack-dev-server^2.9.1 globally, it refused to acknowledge its peer of webpack^3.0.0 despite the fact that it was present. Is that related?</p>