code: def upload_fail(message=None):
      
      if message is None:
          message = current_app.config['CKEDITOR_UPLOAD_ERROR_MESSAGE']
      return jsonify(uploaded=0, error={'message': message})
docs: Return a upload failed response, for CKEditor >= 4.5.
      For example::
  
          from flask import send_from_directory
          from flask_ckeditor import upload_success, upload_fail
          
          app.config['CKEDITOR_FILE_UPLOADER'] = 'upload'  # this value can be endpoint or url
  
  
          @app.route('/files/<path:filename>')
          def uploaded_files(filename):
              path = '/the/uploaded/directory'
              return send_from_directory(path, filename)
  
          @app.route('/upload', methods=['POST'])
          def upload():
              f = request.files.get('upload')
              if extension not in ['jpg', 'gif', 'png', 'jpeg']:
                  return upload_fail(message='Image only!')  # <--
              f.save(os.path.join('/the/uploaded/directory', f.filename))
              url = url_for('uploaded_files', filename=f.filename)
              return upload_success(url=url)
  
      :param message: error message.
  
      .. versionadded:: 0.4.0