Handle undefined webpack build error (#2360)

In some cases, `stats.compilation.errors` contains an `undefined`. In these cases, `toJson` crashes when accessing a method on this undefined object. 
This change now filters out `undefined` errors

packages/gatsby/src/commands/build-html.js

@@ -30,7 +30,7 @@ module.exports = async (program: any) => {
       }
       const outputFile = `${directory}/public/render-page.js`
       if (stats.hasErrors()) {
-        let webpackErrors = stats.toJson().errors
+        let webpackErrors = stats.compilation.errors.filter((e) => e)
         return reject(
           createErrorFromString(webpackErrors[0], `${outputFile}.map`)
         )