From the HttpServletResponse#sendError() javadoc:
The server defaults to creating the response to look like an HTML-formatted server error page containing the specified message, setting the content type to “text/html”, leaving cookies and other headers unmodified. If an error-page declaration has been made for the web application corresponding to the status code passed in, it will be served back in preference to the suggested msg parameter.
So with this approach you have no other option than extracting the message from the HTML response yourself. JSoup may however be useful in this.
To achieve what you want, you need to set the error code and write the response yourself, e.g.
response.setStatus(code);
response.setContentType("application/json");
response.setCharacterEncoding("UTF-8");
response.getWriter().write(json);
Instead of code you could by the way also use one of the HttpServletResponse.SC_XXX constants for this.