Implement 501 status code

[schnuerle]

From @michael-danko-passport

I have been looking for any documentation around the standard API responses and relevant status codes. I know the spec calls out a 501 - Not Implemented for some APIs but if there's some detailed documentation about this I may have overlooked, it should not be hard to also include within the OpenAPI spec. As a result of not finding this documentation, all of the API responses are a work in progress. We can easily apply them to each endpoint if they're structured similarly since these would be shared models.

[michael-danko-passport]

I made sure all of the optional endpoints were documented in #13 but there are no details around how this is defined.