Allow to easily throw error response in ApiJsonResponse<T> methdods

While StatusCode can already be parsed as a response,
it's not always easy to propagate the correct Ok() signature
in a method that only returns errors.

Throw the error variant is trivial, but verbose so an helper method
error_response(status_code, error_message) can be helpful to keep
code easy to read and maintain.

Expected use:

if ... {
    return error_response(StatusCode::..., "...");
}

Test Plan: Tested with Alkane HTTP API server code.

Differential Revision: https://devcentral.nasqueron.org/D4152