404.html: added text to suggest logging in might help

gracedb/templates/404.html

@@ -5,4 +5,5 @@
 
 {% block content %}
     <p>The item you requested was not found.</p>
+    <p>This may be because the item is restricted to be viewed only by authorised persons, if you have authorisation, please log in and try again.</p>
 {% endblock %}