Better raise message

* tiramisu/config.py (Config._valid_len): use master and slave name and
  value.

tiramisu/config.py

@@ -199,8 +199,12 @@ class Config(object):
             return True
         if master_len != len(slave_value):
             master_name = self._cfgimpl_descr.get_master_name()
-            raise ValueError("invalid len for the group of"
-                    " the option {0}".format(slave_name))
+            master_value = self._cfgimpl_values[master_name]
+            raise ValueError("invalid len of '{0}={1}' for the group of"
+                             " '{2}={3}'".format(slave_name,
+                                                 slave_value,
+                                                 master_name,
+                                                 master_value))
 
     def fill_multi(self, name, result, use_default_multi=False, default_multi=None):
         """fills a multi option with default and calculated values