ESLint warnings and errors ignored in npm run build

Created by: mjomble

To reproduce:

create-react-app cra-test
cd cra-test
echo foo() > src/index.js
npm start

Output:

Warning in ./src/index.js

E:\dev\cra-test\src\index.js
  1:1  warning  'foo' is not defined  no-undef

✖ 1 problem (0 errors, 1 warning)

But if you run npm run build:

Creating an optimized production build...
Compiled successfully.

File sizes after gzip:
...

Tested with 0.2.1

The reason is explained here:

to handle all errors and warnings with the node.js API you need to test err, stats.errors and stats.warnings

stats.errors and stats.warnings are checked in start.js but build.js only checks for err.

I may be able to submit a PR for this if needed.