Check as:
public void onReceivedError(WebView view, int errorCode,
String description, String failingUrl) {
Log.e("ProcessPayment", "onReceivedError = " + errorCode);
//404 : error code for Page Not found
if(errorCode==404){
// show Alert here for Page Not found
view.loadUrl("file:///android_asset/Page_Not_found.html");
}
else{
}
}