[#36] Add webfactory/exceptions-bundle to app

* Bundle allows for seeing custom error pages
  in develop rather than just in prod.
[#36] Add 404 and 500 custom error pages

* Add app/Resources/TwigBundle/views/Exception/
  namespace, which Symfony checks for custom
  error pages.
Merge branch 'task-36-customize_404_error_page' into develop
Merge branch 'develop'