HTTP response codes

    1xx informational response – the request was received, continuing process
    2xx successful – the request was successfully received, understood, and accepted
        200 OK
    3xx redirection – further action needs to be taken in order to complete the request
        301 Moved Permanently => Redirection
        304 Not Modified
    4xx client error – the request contains bad syntax or cannot be fulfilled
        400 Bad request
        401 Unauthorized
        403 Forbidden
        404 Not found
    5xx server error – the server failed to fulfil an apparently valid request
        500 Internal Server Error
        502 Bad Gateway - The server was acting as a gateway or proxy and received an invalid response from the upstream server
        503 Service Unavailable - The server cannot handle the request (because it is overloaded or down for maintenance)
        504 Gateway Timeout - The server was acting as a gateway or proxy and did not receive a timely response from the upstream server.
    https://en.wikipedia.org/wiki/List_of_HTTP_status_codes