Error 4xx

This series mainly addresses errors occurring on the client-side.

Bad request- 400

Unauthorized- 401

Forbidden- 403

Not found -404

Conflict-409

Unsupported media type-415

Error 5xx

This series mainly addresses errors occurring on the server-side.

Internal Server Error-500

Not implemented-501

Bad Gateway-502

Service unavailable-503

Gateway timeout-504