zk.loadScript() can check file existence

Requirement Description

When we load an external script like
zk.loadScript('http://ende.de/test.js', 'myscript');

It might fail to load the external script. In such case, the whole application will halt on Processing message and go timeout with the message:
Error code 1: ZK error, before mounting. ...

Although users can see an error message in developer tool. Maybe ZK can check a script's existence before loading.

Current Workaround

Check before loading like:
http://www.kirupa.com/html5/checking_if_a_file_exists.htm
http://stackoverflow.com/questions/3646914/how-do-i-check-if-file-exists-in-jquery-or-javascript

Proposed solution

allow to pass a callback function. ZK calls the callback function after loading is done and pass the loading status no matter succeed or fail.