How to trap CF error inside AJAX function

We are using Coldfusion MX7. I am calling a CFM file in AJAX function. I would like to know if there is a way to display the server-side error inside the AJAX function.

For example, the following is the function called after receiving response from the server (AJAX call).

function loginReply() {
if(http.readyState == 4){
var response = http.responseText;
if(response == 0){
// if login fails

alert(errorDetails); //Can I access the Coldfusion error details here?

} else {
document.getElementById('login_response').innerHTML = response;
}
}
}

Thanks