Inconsistent error message for duplicate user

When duplicate user is added an inconsistent error message to the rest
of the framework is printed. This patch changes this to standard
duplicate error message.

https://fedorahosted.org/freeipa/ticket/1116

ipalib/plugins/user.py

@@ -261,7 +261,7 @@ class user_add(LDAPCreate):
             self.api.Command['group_show'](keys[-1])
             try:
                 self.api.Command['user_show'](keys[-1])
-                raise errors.DuplicateEntry()
+                self.obj.handle_duplicate_entry(*keys)
             except errors.NotFound:
                 raise errors.ManagedGroupExistsError(group=keys[-1])
         except errors.NotFound: