Salesforce HTTP Status code and Error Responses

HTTP Status Code Description
200 “OK” success code, for GET,PATCH 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.
400 The request could not be understood, usually because the ID is not valid for the particular resource. For example, if you use a userId where a groupId is required, the request returns 400.
401 The session ID or OAuth token has expired or is invalid. Or, if the request is made by a guest user, the resource isn’t accessible to guest users. The response body contains the message and errorCode.
403 The request has been refused. Verify that the context user has the appropriate permissions to access the requested data, or that the context user is not an external user.
404 Either the specified resource was not found, or the resource has been deleted.
405 The method specified in the Request-Line isn’t allowed for the resource specified in the URI.
409 A conflict has occurred. For example, an attempt was made to update a request to join a group, but that request had already been approved or rejected.
412 A precondition has failed. For example, in a batch request, if haltOnError is true and a subrequest fails, subsequent subrequests return 412.
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 could not be completed. Contact Salesforce Customer Support.
503 Too many requests in an hour or the server is down for maintenance.