Add 5XX responses to `openapi.yml`

Our OpenAPI specification currently doesn't include 5XX responses, though we definitely return them. Add them. 🙂 See the relevant discussion on the MR. Excerpt:

by definition of "Internal server error" is a 500 a possibility of all server responses? do we need to document that? I am not sure who that helps - @tedbow

Yes, that's true... of course, the intent would be to provide documentation for client developers and some "free" regression testing with debugging functionality during development. Oh, and it's not so much the fact of the responses as the shape of them. - @TravisCarden