Error 400

400 Error : Bad request.

The request could not be understood by the server due to malformed syntax. 
The client should not repeat the request without modifications.

Error 401

401 Error : Access denied.

Error 403

403 Error : Forbidden.

Error 404

404 Error : Not found.

This error may occur if the file that you are trying to access has been moved or deleted.

Error 500

500 Error : Internal server error.

You see this error message for a wide variety of server-side errors.