title: Issue running yarn watch with webpack
body: <p>I am working on a project and using webpack for the first time. I ran into an issue running <code>yarn watch</code>. Here is my code in webpage.config.js:</p>
  
  <pre><code>const path = require('path');
  
  // include the js minification plugin
  const UglifyJSPlugin = require('uglifyjs-webpack-plugin');
  
  // include the css extraction and minification plugins
  const MiniCssExtractPlugin = require("mini-css-extract-plugin");
  const OptimizeCSSAssetsPlugin = require("optimize-css-assets-webpack-plugin");
  
  module.exports = {
    entry: ['./src/js/site.js', './src/scss/style.scss'],
    output: {
      filename: './dist/js/site.min.js',
      path: path.resolve(__dirname)
    },
    module: {
      rules: [
        // perform js babelization on all .js files
        {
          test: /\.js$/,
          exclude: /node_modules/,
          use: {
            loader: "babel-loader",
            options: {
            presets: ['babel-preset-env']
            }
          }
        },
        // compile all .scss files to plain old css
        {
          test: /\.(sass|scss)$/,
          use: [MiniCssExtractPlugin.loader, 'css-loader', 'sass-loader']
        }
      ]
    },
    plugins: [
      // extract css into dedicated file
      new MiniCssExtractPlugin({
        filename: './dist/css/style.min.css',
        path: path.resolve(__dirname)
      })
    ],
    optimization: {
      minimizer: [
        // enable the js minification plugin
        new UglifyJSPlugin({
          cache: true,
          parallel: true
        }),
        // enable the css minification plugin
        new OptimizeCSSAssetsPlugin({})
      ]
    }
  };
  </code></pre>
  
  <p>In running this I get a couple of errors. The first says:</p>
  
  <blockquote>
    <p>Insufficient number of arguments or no entry found</p>
  </blockquote>
  
  <p>and then another one comes up that says:</p>
  
  <blockquote>
    <p>ERROR in Entry module not found: Error: Can't resolve './src' in 'FOLDER_NAME_HERE' error Command failed with exit code 2.</p>
  </blockquote>
  
  <p>Here is my package.json file as well:</p>
  
  <pre><code>{
    "name": "project name",
    "version": "1.0.0",
    "description": "project description",
    "scripts": {
      "build": "webpack --mode development",
      "dist": "webpack --mode production",
      "watch": "webpack --watch --mode development"
    },
    "devDependencies": {
      "babel-core": "^6.26.3",
      "babel-loader": "^8.0.4",
      "babel-preset-env": "^1.7.0",
      "css-loader": "^1.0.0",
      "imagemin-cli": "^3.0.0",
      "mini-css-extract-plugin": "^0.4.4",
      "node-sass": "^4.9.3",
      "npm-run-all": "^4.1.3",
      "optimize-css-assets-webpack-plugin": "^5.0.1",
      "sass-loader": "^7.1.0",
      "uglifyjs-webpack-plugin": "^2.0.1",
      "webpack": "^4.20.2",
      "webpack-cli": "^3.1.2"
    },
    "author": "Me",
    "license": "MIT"
  }
  </code></pre>
  
  <p>I did a little searching and found some stuff on Github regarding this, but no real solutions. I have a '/src' folder so it makes no sense for an error to say it can't resolve './src' unless that file path is incorrect (I've tried all sort of other possibilities just to test it, what I have now is what it was originally). Any help is greatly appreciated.</p>