when build is failing due to ESLint errors, filter warnings out

[milocosmopolitan]

@gaearon I wasn't sure if you wanted to place process.exit(1) when there are warning or not, but if process.env.CI hasn't set it should print warnings. I've tested by enabling root/.eslintignore line4 packages/react-scripts/template and produce some warning.

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[gaearon]

Hey, thanks for the PR!
I'm sorry I didn't review it in time.

I don't think this does what I wanted. I didn't really do a great job of explaining 😞
I sent #2120 instead which should fix the issue, but I appreciate the effort that went into this!