add 'obs-id' as mandatory filter field

ldvspec/lofardata/templates/lofardata/workspecification/create_update.html

@@ -204,7 +204,11 @@
                             The input is invalid:
                             {% for field, errors in form.errors.items %}
                                 {% for error in errors %}
-                                    <li class="text text--red text--faded">{{ field }}: {{ error }}</li>
+                                    {% if field == '__all__' %}
+                                        <li class="text text--red text--faded">{{ error }}</li>
+                                    {% else %}
+                                        <li class="text text--red text--faded">{{ field }}: {{ error }}</li>
+                                    {% endif %}
                                 {% endfor %}
                             {% endfor %}
                         </div>