How to display error messages when client-side validation fails?

Our jQuery library supports a client-side validation for uploads, including file types, maximum file size, and more.
For more information:
https://github.com/cloudinary/cloudinary_js#client-side-image-resizing-before-upload

After adding your required validation (e.g., acceptFileTypes, maxFileSize, etc) jquery-file-upload does the validation, however it doesn't print the message for you. You'll need to specify how the error message will be shown.
One way to do it is to add the following to pop-up window with the error type:

processalways: function(e,data){
  if (data.files.error) alert(data.files[0].error);
}