API should use a error serializers for documenting error responses instead of an error class

[obulat]

Description

The API does not set up the error response documentation correctly.

Reproduction

1. Run the api locally using `just api/up`
2. Look at the API logs.
3. See warnings.

/api/api/views/oauth2_views.py: Warning [CheckRates]: could not resolve "<class 'rest_framework.exceptions.PermissionDenied'>" for GET /v1/rate_limit/. Expected either a serializer or some supported override mechanism. Defaulting to generic free-form object.
4. Go to localhost:50280, and look at the documentation for error responses. See that they use a generic object instead of an object that has the same shape as our error responses:

Additional context

Noticed during review of #4126