Check ESLint warnings and errors in npm run build

Created by: mjomble

Fixes #440 (closed)

Test plan:

echo foo() > template/src/index.js
npm run build

Expected output:

Creating an optimized production build...
Compiled with warnings.

Warning in ./template/src/index.js

E:\dev\create-react-app\template\src\index.js
  1:1  warning  'foo' is not defined  no-undef

✖ 1 problem (0 errors, 1 warning)


You may use special comments to disable some warnings.
Use // eslint-disable-next-line to ignore the next line.
Use /* eslint-disable */ to ignore all warnings in a file.

followed by npm errors.

Should not show "Compiled successfully." message.

Also, when running npm start and there are warnings, but the server is still started, this should appear after the warnings:

The app is running at:

  http://localhost:3000/

Note that the development build is not optimized.
To create a production build, use npm run build.