You can use OnClientFileUploadFailed
event of Telerik RadAsync Upload.
Something like
function OnClientFileUploadFailed(sender, args) {
var upload = $find("<%= RadUpload.ClientID %>");
var errormsg = args.get_message();
var displaymsg = new String();
sender.deleteFileInputAt(0);
if (errormsg.search("[IO.IO_SharingViolation_File]") != -1) {
displaymsg = "File: is currently in use. Please close the file and try again.";
}
else {
displaymsg = "The file you selected is currently in use. Please close the file and try again.";
}
alert(displaymsg);
args.set_handled(true);
}