Send error message as JSON object

I have two servlet: first servlet is similar to a client and creates an HttpURLConnection to call the second servlet.

I would like send a special error, formatted like a JSON object, so I call sendError method in this way:

response.sendError(code, "{json-object}")

But in the first servlet when I read error with getResponseMessage method I just get standard HTTP message and not my json object as a string.

How I can get my json string?