Ugly build message on master

[Timer]

While testing PWA I ran into this:

Joes-MacBook-Pro:website joe$ yarn build
yarn build v0.24.4
$ react-scripts build 
Creating an optimized production build...
Total precache size is about 3.15 MB for 11 resources.
Failed to compile.

9:0-8 "export 'register' was not found in './register'

Looks fine when running yarn start:

[gaearon]

In general I think we should reformat common Webpack errors and show them with better messages and more instructions on how to fix (e.g. "this error is commonly caused by [...]")