title: Odoo - raise javascript warning
body: <p>How to raise a javascript warning in Odoo?</p>
  
  <p>There is an example in /addons/web_view_editor/static/src/js/view_editor.js:</p>
  
  <pre><code>this.do_warn(_t("The following fields are invalid :"), msg)
  </code></pre>
  
  <p>How to call similar function by <code>on_change</code> method, for instance, in python?</p>