Add invariant for value returned when user modifies the Webpack config (#300)

Fixes #269

Author: KyleAMathews

lib/utils/webpack.config.js

@@ -5,6 +5,7 @@ import Config from 'webpack-configurator'
 const debug = require('debug')('gatsby:webpack-config')
 import path from 'path'
 import _ from 'lodash'
+import invariant from 'invariant'
 
 import babelConfig from './babel-config'
 
@@ -380,7 +381,14 @@ module.exports = (program, directory, stage, webpackPort = 1500, routes = []) =>
   })
 
   if (modifyWebpackConfig) {
-    return modifyWebpackConfig(module(config), stage)
+    const modifiedWebpackConfig = modifyWebpackConfig(module(config), stage)
+    invariant(_.isObject(modifiedWebpackConfig),
+              `
+              You must return an object when modifying the Webpack config.
+              Returned: ${modifiedWebpackConfig}
+              stage: ${stage}
+              `)
+    return modifiedWebpackConfig
   } else {
     return module(config)
   }