Throw non-zero exit code if webpack build fails

Not only `err`, `stats.hasErrors` also should be handled.
I found this bug in [Pipeline 42881 build] - some errors exist,
but build result was successful.

See also: https://webpack.js.org/api/node/#error-handling

[Pipeline 42881 build]: https://git.pleroma.social/pleroma/pleroma-fe/-/jobs/225066

build/build.js

@@ -36,4 +36,8 @@ webpack(webpackConfig, function (err, stats) {
     chunks: false,
     chunkModules: false
   }) + '\n')
+  if (stats.hasErrors()) {
+    console.error('See above for errors.')
+    process.exit(1)
+  }
 })