Make 404 page more generic (temporary)

gracedb/templates/404.html

 {% extends "base.html" %}
 
-{% block title %}404 – Not Found{{ object.graceid }}{% endblock %}
-{% block heading %}Not Found {{ object.graceid }}{% endblock %}
+{% block title %}404 – Not Found{% endblock %}
+{% block heading %}Not Found{% endblock %}
 
 {% block content %}
     <p>The item you requested was not found.</p>
-    {{ message|safe }}
 {% endblock %}