Logged out warnings:
- Don't remove login error messages coming from wp_signon().
- When the login form is shown in iframe, open all links in a new tab/window.
- Add filter for the login form error message.
See #23295

git-svn-id: http://core.svn.wordpress.org/trunk@24179 1a063a9b-81f0-0310-95a4-ce76da25c4cd