Webpack warnings trigger build errors when running on CI

Created by: scholtzm

(Initial discussion starts here.)

Pull request https://github.com/facebookincubator/create-react-app/pull/944 adds a new behaviour related to CIs - crash the build during CI whenever linter warnings are encountered.

This PR however introduced new side effect - webpack warnings will also crash the build, e.g.:

This seems to be a pre-built javascript file. Though this is possible, it's not recommended. Try to require the original source to get better results.

We could consider this particular warning to be harmless. Since we have no access to webpack config, we cannot use module.noParse option either.