EZP-21939: eZ Find should return errors when Solr is down

JIRA: https://jira.ez.no/browse/EZP-21939

If SOLR server is not available, return an actual error, which makes it possible to handle the condition further.
In practice, this changes the returned HTTP status (200 => 404), but the behavior can be further customized in error.ini ```[ErrorSettings]```

Testing:  manual