Display custom 404 page: invalid route / record not found - Bounty #428

[Custom 404 page not being displayed when visiting an invalid path](https://assembly.com/coderwall/bounties/428)
---
- rescue_from ActionController::RoutingError doesn’t do anything
- `unless Rails.env.production?` made record not found work
- added tests! :-)
- new ErrorsController
- handle 422 & 500 errors with public pages
- annotate routes

:information_source: The reason the error paths are not at the bottom of the routes file is because the /:username route :smile_cat: 

To test in development: 
https://github.com/assemblymade/coderwall/blob/master/config/environments/development.rb#L13
```ruby
#config/environments/development.rb
config.consider_all_requests_local            = false
```