Merge pull request #251 from lucasgranberg/patch-1

Throw exception when trying to use export functionallity.

Model/ModelManager.php

@@ -545,12 +545,10 @@ public function collectionRemoveElement(&$collection, &$element)
 
     /**
      * {@inheritDoc}
-     *
-     * @return null Not really implemented.
      */
     public function getDataSourceIterator(DatagridInterface $datagrid, array $fields, $firstResult = null, $maxResult = null)
     {
-        return null;
+        throw \RuntimeException("Datasourceiterator not implemented.");
     }
 
     /**