Ensure eslint-loader works in a sensible way in prod

[gaearon]

I haven’t actually checked what it does.
I think we don’t want to fatal on build, but we want to print any errors/warnings.
We also want to make sure it doesn’t include console.log()s with those warnings into prod bundle.

[gaearon]

Currently it is silent about lint warnings during build.
Not a big deal probably. We can enable special lint for build later.