REST HTTP Error Codes : Nice Points .

HTTP Error Codes:

When errors occur, the response header contains an HTTP code, and the response body
typically contains the error code, the message accompanying that error, and the field or
object where the error occurred.

300
The value used for an external ID exists in more than one
record. The response body contains the list of matching
records.

400
The request could not be understood, usually because the
JSON or XML body has an error.

401 The session ID or OAuth token used has expired or is invalid.
The response body contains the message and errorCode.

403 The request has been refused. Verify that the logged-in user
has appropriate permissions.

404 The requested resource could not be found. Check the URI for
errors, and verify that there are no sharing issues.

405 The method specified in the Request-Line is not allowed for
the resource specified in the URI.

415 The entity specified in the request is in a format that is not
supported by specified resource for the specified method.

500 An error has occurred within Force.com, so the request could
not be completed. Contact salesforce.com Customer Support.