Missing uploadBtn-reference on a couple of callbacks

[magnus-eriksson]

Is there a reason why the callbacks onExtError() and onSizeError() doesn't get the button reference passed to them?

If it could be added, it would be awesome. Would really need them on those callbacks as well.

[emzyme20]

This would be super helpful as when you initialise the uploader with an array of buttons, you have no idea which target caused the error.