fail tests on lint warnings

[mythmon]

No description provided.

[mbostock]

I think we want yarn test to fail locally, too, which means import order should be an error rather than a warning.

[mythmon]

I personally get a lot of value from separating lints into correctness and style, where correctness is errors and style is warnings. It lets me ignore the style errors while I'm working and not break flow to figure out if I need to deal with an error right now or not. I can make yarn test fail either way though.