Fixed InvalidClassnameError

InvalidClassnameError was throwing a NameError, as 'self.' was missing when trying to access the class name.
diff --git a/reclass/errors.py b/reclass/errors.py
index ddb95fd..5ce4d73 100644
--- a/reclass/errors.py
+++ b/reclass/errors.py
@@ -195,7 +195,7 @@
 
     def _get_message(self):
         msg = "Invalid character '{0}' in class name '{1}'."
-        return msg.format(self._char, classname)
+        return msg.format(self._char, self._classname)
 
 
 class DuplicateNodeNameError(NameError):