Fix warning message for unregistering AST transformers.

IPython/core/interactiveshell.py

@@ -2691,7 +2691,7 b' class InteractiveShell(SingletonConfigurable):'
             try:
                 node = transformer.visit(node)
             except Exception:
-                warn("AST transformer %r threw an error. It will be unregistered.")
+                warn("AST transformer %r threw an error. It will be unregistered." % transformer)
                 self.ast_transformers.remove(transformer)
         
         return node