Javascript: Error in FTP access

The following is a routine to download content of a file.

Code:

---

var req = new XMLHttpRequest();
req.open( 'GET', 'ftp://username:[email protected]/index.html', true );

req.onreadystatechange = function() {
  if( req.readyState == 4 ) {
    print( req.responseText );
  }
}
req.send( null );

---

It works fine except for the error handling. When login info is incorrect the following generic javascript window pops up.

Code:

---

530 Login Incorrect

---

When the file does not exits, it will pop up the following.

Code:

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550/index.html No such file or directory

---

What I want to do is to suppress the generic error window and its message with my own. So how do I capture the type of error (error 530, 550, etc), and how do I prevent the generic javascript erro window from popping up?

You'd probably be better off asking this in a web developer community.

EDIT: But AFAIK, there is no error trapping in JS.

Yes, there is. Since 1.4, I think, JavaScript supports try ... catch.

But it won't work in this situation, because that FTP handling is done internally.

I won't even talk about the absurdity of using XMLHttpRequest to get a file that is neither XML nor retrieved over HTTP.