I use dropzone with CI, i don\'t know how to display error message and custom message when upload false, this is my script
Dropzone.autoDiscover = false;
For anyone in need:
You can return a response message from the server using echo. Then in the js code add an error event handler
PHP
header("HTTP/1.0 400 Bad Request");
echo "Ups error message";
JS
this.on('error', function(file, response) {
$(file.previewElement).find('.dz-error-message').text(response);
});