Attempt at porting MagentoBetter404 to Magento 2.

Currently globally listens for layout_generate_blocks_after events, checks if the status of the response is 404 and then replaces the content of the cms_page block with a custom block.

Notes

?? Maybe the whole NoRoute (NoRouteHandler) system of Magento 2 should be hooked into to achieve this instead. -> see noroute branch