Show a proper error message on the dashboard when the current theme is broken.

Clean up Appearance -> Themes when we don't have enough information to show for a broken theme.

Clean up broken/ugly "alt" row styling when we are displaying broken themes.

props johnbillion for initial patch.
fixes #21670.

Built from https://develop.svn.wordpress.org/trunk@25666


git-svn-id: http://core.svn.wordpress.org/trunk@25582 1a063a9b-81f0-0310-95a4-ce76da25c4cd