Tweak `ResponseStatus` enum to align with standard HTTP, or replace with an extant Flask enum

[LswaN58]

Currently, we're using base 200, 400, and 500 as response values, which is in line with our early "just emulate the standards to enable quick, collaborative prototyping" approach, but should now actually align to actual standards.

Alternately, if Flask provides a nice, usable enum for this, we'll just use that.

[LswaN58]

On further looking into this, it kinda seems like we're... not that far off from the actual standards. There's not really HTTP codes for a lot of things that make sense for the application to communicate via response code. Maybe I'm just a tad too far removed from studying the network layer architecture.

Anyway, we're going to add option for 404 responses for cases where e.g. the given game ID was not a real game we have, but beyond that we can pretty much stick with generic 400/500 errors.

[LswaN58]

After some more time and use cases, we want to add a couple more status codes:

• 201 : Created

  The request succeeded, and a new resource was created as a result. This is typically the response sent after POST requests, or some PUT requests.
  We'll want this available as an option for any future API work involving PUT/POST requests.

• 204 : No Content

  There is no content to send for this request, but the headers are useful. The user agent may update its cached headers for this resource with the new ones.
  We'll use this e.g. for endpoints that we've set up as placeholders but haven't fully implemented yet.