* compile _document using webpack
* don't emit the bundle file of _document.js
* exclude _document.js from minChunks of CommonsChunkPlugin
* handle creation/removal of pages/_document.js
* improve path handlings
* add detach-plugin
* detach-plugin: remove unused property
* watch-pages-plugin: replace _error.js when user defined one was added/removed
* dynamic-entry-plugin: delete cache
* fix HMR settings for _error.js
* render: pass error only on dev
* hot-reload: enable to hot-reload error page
* server: check if /_error has compilation errors
* webapck-dev-client: fix reloading /_error