Login/UnlockDialog: don't reset immediately if auth fails

Instead of showing a notification, add a small message immediately
below the entry, and give the user two more attempts to login,
before going back to the welcome or lock screen.

https://bugzilla.gnome.org/show_bug.cgi?id=682544

data/theme/gnome-shell.css

@@ -2189,6 +2189,10 @@ StButton.popup-menu-item:insensitive {
     background-gradient-end: #74a0d0;
 }
 
+.login-dialog-message-warning {
+    color: orange;
+}
+
 .unlock-dialog-user-name-container {
     spacing: .4em;
 }