Implement errorhandling for misconfigured fields

If a field configured to be a text field, but in the datastore it is actually a resource, we provoke:
Error: Call to undefined method EasyRdf\Resource::getLang() in Drupal\wisski_adapter_sparql11_pb\Plugin\wisski_salz\Engine\Sparql11EngineWithPB->pathToReturnValue() (line 1523 of /opt/drupal/web/modules/contrib/wisski/wisski_adapter_sparql11_pb/src/Plugin/wisski_salz/Engine/Sparql11EngineWithPB.php)

We should handle errors like this more meaningful.