Add Error code for chunk loading failure

[SergeyPanchenkoWeWork]

Feature request

Added code property on Error object received when failed to load chunk, To help identify loading errors.
Related to webpack/webpack#8695

What is the expected behavior?
add CSS_CHUNK_LOAD_FAILED in error.code

How should this be implemented in your opinion?
Add error.code = 'CSS_CHUNK_LOAD_FAILED' in https://github.com/webpack-contrib/mini-css-extract-plugin/blob/master/src/index.js#L362

[alexander-akait]

PR welcome

[SergeyPanchenkoWeWork]

Sure #347 😃