Allow showing warnings from underlying libraries

[bebraw]

The problem

Recently our project was bitten by a known issue in styled-components (styled-components/styled-components#2962 (comment)) that caused our production build to lose all its styling.

The issue itself had to do with the fact that we used an @import to refer to a font and currently that's not supported when building. styled-components gives a build-time warning so developers know to fix this. Personally I would have thrown an Error as it's a fatal issue but they went this way.

Unfortunately, the way our styleguidist is configured, we weren't able to see the warning in time. I tried configuring webpack to emit more but it didn't seem to make a difference.

Proposed solution

Ideally there should be a way to show webpack warnings in styleguidist output to catch these kind of issues.

Alternative solutions

Additional context

[sapegin]

Could you try in #1553? I think I did some changes there that may help to see more errors.