fetch http client return type is wrong when `unwrapResponseData` is false

Looking at this code: https://github.com/acacode/swagger-typescript-api/blob/master/templates/base/http-clients/fetch-http-client.ejs#L159-L224

The return type should be `Promise>` when `unwrapResponseData` is false. If the request is successful and no error is thrown, the error property will always be null.

In fact, it seems like there is no point in having the generic `E` as a part of custom fetch because there is no guarantee that it will be the type given this line https://github.com/acacode/swagger-typescript-api/blob/master/templates/base/http-clients/fetch-http-client.ejs#L206.