[DoctrineBridge] Improve exception message at IdReader::getIdValue()

src/Symfony/Bridge/Doctrine/Form/ChoiceList/IdReader.php

@@ -95,10 +95,7 @@ class IdReader
         }
 
         if (!$this->om->contains($object)) {
-            throw new RuntimeException(
-                'Entities passed to the choice field must be managed. Maybe '.
-                'persist them in the entity manager?'
-            );
+            throw new RuntimeException(sprintf('Entity of type "%s" passed to the choice field must be managed. Maybe you forget to persist it in the entity manager?', get_class($object)));
         }
 
         $this->om->initializeObject($object);