[tree] fix the exception message for tree repository

lib/Gedmo/Tree/Entity/Repository/AbstractTreeRepository.php

@@ -35,7 +35,7 @@ abstract class AbstractTreeRepository extends EntityRepository
         }
 
         if (is_null($treeListener)) {
-            throw new \Gedmo\Exception\InvalidMappingException('This repository can be attached only to ORM tree listener');
+            throw new \Gedmo\Exception\InvalidMappingException('Tree listener was not found on your entity manager, it must be hooked into the event manager');
         }
 
         $this->listener = $treeListener;
@@ -51,4 +51,4 @@ abstract class AbstractTreeRepository extends EntityRepository
      * @return bool
      */
     abstract protected function validate();
-}
+}