Live validation doesn't properly work when creating new pages and users

Steps to reproduce:

1. Create a new page called "test" in the Main space
2. Try to create another page called still "test" in the same space

You'll get an error saying: "The page Main.test already exists. Please fill in a new page name.", but when you type in the name for the second time there's a green "Ok" corresponding to the Page Name field.

The same bug can be reproduced in the Users section of the Administration or on http://localhost:8080/xwiki/bin/register/XWiki/Register. Try to create the same username twice. You'll get an error: "User already exists." only if you click on Register. When you type in the username for the second time there's a green "Ok".