feat: added error code to chunk load Error (#347)

Author: SergeyPanchenkoWeWork

Diff for: src/index.js

@@ -359,6 +359,7 @@ class MiniCssExtractPlugin {
                   Template.indent([
                     'var request = event && event.target && event.target.src || fullhref;',
                     'var err = new Error("Loading CSS chunk " + chunkId + " failed.\\n(" + request + ")");',
+                    'err.code = "CSS_CHUNK_LOAD_FAILED";',
                     'err.request = request;',
                     'delete installedCssChunks[chunkId]',
                     'linkTag.parentNode.removeChild(linkTag)',