Wave 02: Read One Book and 404s

RESTful Endpoint(s): Read One Planet

Create the following endpoint(s), with similar functionality presented in the Hello Books API:

As a client, I want to send a request...

...to get one existing planet, so that I can see the id, name, description, and other data of the planet.
... such that trying to get one non-existing planet responds with get a 404 response, so that I know the planet resource was not found.
... such that trying to get one planet with an invalid planet_id responds with get a 400 response, so that I know the planet_id was invalid.