Error codes for http requests

HTTP response code	Description
200	“OK” success code, for GET or HEAD request.
201	“Created” success code, for POST request.
204	“No Content” success code, for DELETE request.
300	The value returned when an external ID exists in more than one record. The response body contains the list of matching records.
304	The request content has not changed since a specified date and time. The date and time is provided in a If-Modified-Since header. See Get Object Metadata Changes for an example.
400	The request couldn’t be understood, usually because the JSON or XML body contains an error.
401	The session ID or OAuth token used has expired or is invalid. The response body contains the message anderrorCode.
403	The request has been refused. Verify that the logged-in user has appropriate permissions.
404	The requested resource couldn’t be found. Check the URI for errors, and verify that there are no sharing issues.
405	The method specified in the Request-Line isn’t allowed for the resource specified in the URI.
415	The entity in the request is in a format that’s not supported by the specified method.
500	An error has occurred within Force.com, so the request couldn’t be completed. Contact Salesforce Customer Support.