Warn use if filter method is called without saveFilter

[garak]

If you forget calling saveFilter and call filter directly, the $forms property in the service is empty, causing a notice in the filter method and making it not work.
We should add a check and raise an exception to ease debugging.

Good example:

<?php
public function action(Filter $filter): Response
{
   $filter->saveFilter(MyFilterType::class, 'my_name');
   $filters = $filter->filter('my_name');
   // etc...   
}

Bad example:

<?php
public function action(Filter $filter): Response
{
   $filters = $filter->filter('my_name'); // <-- forgot saving the filter, not working!
   // etc...   
}