SUMMARY OF HTTP ERRORS

Error codes are generated when a request from your browser cannot be fulfilled
  The two main types of error codes are server error (the 500 series),
  and client or browser error codes (the 400 series).

  The values of some of the more common numeric status codes are as follows:

•400 -- Bad request. The request had bad syntax or was inherently impossible to be satisfied.

•401 -- Unauthorized. The authorization header is missing or unacceptable.

•402 -- Payment Required. The parameter to this message gives a specification of
           charging schemes acceptable. The client may retry the request with a suitable ChargeTo header.

•403 -- Forbidden. The request is for something forbidden. Authorization will not help.

•404 -- Not found. The server has not found anything matching the URL given.

•500 -- Internal Error. The server encountered an unexpected condition
           which prevented it from fulfilling the request.

•501 -- Not implemented. The server does not support the facility required.

•502 -- Service temporarily overloaded. The server cannot process the request due to a high load.

•503 -- Gateway Timeout. The response from an intermediate computer was not received quickly
           enough or within a time that the gateway was prepared to wait.