Menu Customizer: if the add-menu-item ajax's returned markup doesn't seem to contain the menu item id where it should be, bail and remove the added-item placeholder. No warning/error message for now, although that may be necessary if this situation pops up a lot in the future (it's currently happening due to issues with adding custom links). Update to-dos.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@935458 b8457f37-d9ea-0310-8a92-e5e31aec5664