Error handling when downloading a file from a servlet - ajax

Question

I have a web application that must work with IE7 (yeah i know..) where the frontend is entirely made with ExtJS4, and theres a servlet used to download files. To download a file i send some parameters so i cant simply use location.href. it must be a POST.
So far it works, but when an exception is thrown in the servlet i dont know how to handle it to show the user some alert box or some message without redirecting to another page.
In my webapp im also using DWR and im aware of the openInDownload() function, but it triggers a security warning in IE.
So, (finally!) the question is
Using this code:
post = function (url, params) {
var tempForm=document.createElement("form");
tempForm.action=url;
tempForm.method="POST";
tempForm.style.display="none";
for(var x in params) {
// ...snip boring stuff to add params
}
document.body.appendChild(tempForm);
tempForm.submit();
return tempForm;
}
is it possible to stay in the same page after submitting ?
or with this other one:
Ext.Ajax.request({
url: './descargaArchivoNivs',
method: 'POST',
autoAbort: true,
params: {
nivs: jsonData
},
success: function(response){
// HERE!!
// i know this is wrong
document.write('data:text/plain,' + response.responseText );
/* this looked promising but a warning pops up
var newwindow = window.open();
newwindow.document.open();
newwindow.document.write('data:text/plain, ' + response.responseText );
newwindow.document.close();*/
},
failure: function(resp){
alert('There was an error');
}
});
is it possible to open the file download dialog // HERE!! with the response content??
or is there some other way to open the file download dialog on success, and on failure show a friendly message without losing the users input (the params of the POST) ?
(sorry if this post was too long)