When an error happens in the XData server while processing a client request, the server might provide information about the error in the HTTP message body as a single JSON object, with a single name/value pair named "error" which value is also a JSON object. Such inline JSON object might contain the following name/value pairs:
"code", which provides a JSON string representing the error code for the error raised by the server;
"message": which provides a JSON string with a human-readable text describing the error.
"message": "Requested entity does not exist."