HTTP status codes are divided into five classes:
1xx (Informational): Request received, continuing process
2xx (Success): The action was successfully received, understood, and accepted
3xx (Redirection): Further action must be taken to complete the request
4xx (Client Error): The request contains bad syntax or cannot be fulfilled
5xx (Server Error): The server failed to fulfill an apparently valid request Thus, the series that relate to errors are:
400 series (Client Error)
500 series (Server Error)
Reference:
RFC 7231, Section 6: HTTP/1.1 Semantics and Content