Add `rambulance` gem to handle errors

[yasulab]

You can quickly check error pages in development mode by accessing:

• http://localhost:3000/rambulance/not_found -> app/views/errors/not_found.html.haml
• http://localhost:3000/rambulance/forbidden -> app/views/errors/forbidden.html.haml
• http://localhost:3000/rambulance/bad_request -> app/views/errors/bad_request.html.haml
• Added test: e2bbeda2 Request for 404 shows 404 page

cf. https://github.com/yuki24/rambulance#readme

Before <---> After