Karma runner: handle preprocessor errors from karma-webpack

[nicojs]

Preprocessor magic in karma is doing more stuff nowadays. For example, the karma-webpack plugin runs entire webpack. Whenever an error is thrown, it is often logged to the console and proceeded, like business as usual. In karma-webpack's case, this means a timeout, because karma is blocked until the next succesful bundle.

In order to fix this issue, these 2 things need to be fixed first:

1. The karma-webpack plugin doesn't handle errors properly. Instead of reporting preprocessor errors to karma, it just logs it to console and proceeds:
   https://github.com/webpack-contrib/karma-webpack/blob/3cc35b33e14405e65b8f91505f212b0b035f8cc0/src/karma-webpack.js#L130-L139
2. Even if the karma-webpack plugin would report the error properly, karma wouldn't handle it properly. I've opened an issue here: Runner middleware does not handle file-list rejections karma-runner/karma#3396

[stale]

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