Fix memeorydb

It's not with the other backends so I missed fixing its addclass to
provide more explicit message.

Author: rouilj

test/memorydb.py

@@ -301,7 +301,7 @@ def __getattr__(self, classname):
     def addclass(self, cl):
         cn = cl.classname
         if cn in self.classes:
-            raise ValueError(cn)
+            raise ValueError(_('Class "%s" already defined.'%cn))
         self.classes[cn] = cl
         if cn not in self.items:
             self.items[cn] = cldb()