The prepares for next-server.
I also took this as an opportunity to get all build directory paths from a single location, as they were previously scattered across webpack/babel plugins and loaders.
* Handle production errors correctly
* Improved source map support
* Make react-hot-loader hold state again
* Remove console.log
* Load modules on demand
* Catch errors in rewriteErrorTrace
* Update comment
* Update comment
* Remove source-map-support
* Load modules in next-dev
* Make sure error logged has sourcemaps too
* Add tests for production runtime errors
* Add tests for development runtime errors. Fix issue with client side errors in development
* Move functionality back to renderError now that error handling is consistent
* Rename to applySourcemaps
