okay the following would work, just extract from the source:
success: function(file, response){
if(response.code == 501){ // succeeded
return file.previewElement.classList.add("dz-success"); // from source
}else if (response.code == 403){ // error
// below is from the source code too
var node, _i, _len, _ref, _results;
var message = response.msg // modify it to your error message
file.previewElement.classList.add("dz-error");
_ref = file.previewElement.querySelectorAll("[data-dz-errormessage]");
_results = [];
for (_i = 0, _len = _ref.length; _i < _len; _i++) {
node = _ref[_i];
_results.push(node.textContent = message);
}
return _results;
}
}