Tag chunk load error with ChunkLoadError

[jorrit]

The Webpack runtime sets the name of chunk load errors to ChunkLoadError (see for example https://github.com/webpack/webpack/blob/1f13ff9fe587e094df59d660b4611b1bd19aed4c/lib/css/CssLoadingRuntimeModule.js#L350).

My suggestion is to also add this to the error thrown by mini-css-extract-plugin (around

mini-css-extract-plugin/src/index.js

Line 877 in d5e540b

'err.code = "CSS_CHUNK_LOAD_FAILED";',

).

[jorrit]

Thanks!