make HttpError valid response type

[fraser-langton]

[DRAFT]

Initial pass at #1507 - just to show it is possible, and rather easy

from ninja import NinjaAPI, Schema
from ninja.errors import HttpError
from ninja.responses import codes_2xx

api = NinjaAPI()


class RequestErrorResponse(Schema):
    detail: str
    reason: list[str]


class ServerErrorResponse(Schema):
    detail: str


class BasicHttpError(HttpError): ...


class RequestError(HttpError[RequestErrorResponse]):
    status_code = 400
    message = "Request Error"


class ServerError(HttpError[ServerErrorResponse]):
    status_code = 500
    message = "Server Error"


@api.get(
    "/excs_in_responses",
    response={codes_2xx: str, 400: RequestError, 404: BasicHttpError, 500: ServerError},
)
def excs_in_responses(request): ...

The main concern I can think of if the disconnect between an exception and the response from the exception (which is done by the exception handler). I think this could be improved by making the HttpException class generate the Response in a method - either by making it a pydnatic model (this might be messy when combined with exception) or continuing with my implementation to use the Schema as a TypeVar (similar to pydantic Generic)

class HttpErrorResponse(BaseModel):
    detail: str


T = TypeVar("T", bound=HttpErrorResponse)


class HttpError(Exception, Generic[T]):
    __response_schema__ = HttpErrorResponse

    @property
    def response(self) -> T:
        return self.__response_schema__(detail=self.message)