[BUG] [typescript-fetch] Missing 4XX and 5XX response

Description

I have an openapi spec for authentification with few responses types:

"/authentication": {
  "post": {
    "tags": [
      "authenticate_user"
    ],
    "operationId": "authenticate",
    "requestBody": {
      "content": {
        "application/json": {
          "schema": {
            "$ref": "#/components/schemas/Auth"
          }
        }
      },
      "required": true
    },
    "responses": {
      "200": {
        "description": "Returns current authentication state.",
        "content": {
          "application/json": {
            "schema": {
              "$ref": "#/components/schemas/AuthResponse"
            }
          }
        }
      },
      "4XX": {
        "description": "",
        "content": {
          "application/json": {
            "schema": {
              "$ref": "#/components/schemas/ErrorResponseObject"
            }
          }
        }
      },
      "5XX": {
        "description": "",
        "content": {
          "application/json": {
            "schema": {
              "$ref": "#/components/schemas/ErrorResponseObject"
            }
          }
        }
      }
    }
  }
}

The AuthResponse & ErrorResponseObject are correctly created, but ErrorResponseObject is not returned in case of 4XX or 5XX errors.

This is the generated file:

import * as runtime from '../runtime';
import type {
  Auth,
  AuthResponse,
  ErrorResponseObject,
} from '../models';
import {
    AuthFromJSON,
    AuthToJSON,
    AuthResponseFromJSON,
    AuthResponseToJSON,
    ErrorResponseObjectFromJSON,
    ErrorResponseObjectToJSON,
} from '../models';

export interface AuthenticateRequest {
    auth: Auth;
}

export class AuthenticateUserApi extends runtime.BaseAPI {
    async authenticateRaw(requestParameters: AuthenticateRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<AuthResponseErrorResponseObject>> {
        if (requestParameters.auth === null || requestParameters.auth === undefined) {
            throw new runtime.RequiredError('auth','Required parameter requestParameters.auth was null or undefined when calling authenticate.');
        }

        const queryParameters: any = {};

        const headerParameters: runtime.HTTPHeaders = {};

        headerParameters['Content-Type'] = 'application/json';

        const response = await this.request({
            path: `/authentication`,
            method: 'POST',
            headers: headerParameters,
            query: queryParameters,
            body: AuthToJSON(requestParameters.auth),
        }, initOverrides);

        return new runtime.JSONApiResponse(response, (jsonValue) => AuthResponseFromJSON(jsonValue));
    }

    async authenticate(requestParameters: AuthenticateRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<AuthResponse> {
        const response = await this.authenticateRaw(requestParameters, initOverrides);
        return await response.value();
    }

}

As you can see ErrorResponseObject is imported but never used.

This creates multiple issues down the line. The main one is that I cannot use the ErrorResponseObject attributes to let the user know what the error actually is.

Start from the generated AuthenticateUserApi example and trace how the OpenAPI 4XX and 5XX responses are represented in the typescript-fetch generator. Add regression coverage for an operation with those responses, and consider the work complete when the generated client exposes and parses ErrorResponseObject for both status ranges.