Theme Customizer: Validate themes with more than just an existence check.

* The current theme goes through validate_current_theme().
 * If doing a preview of a different theme, we check theme->errors().

Also:
 * Don't attach previewing hooks when previewing the current theme.
Aside from being unnecessary, this prevents issues with a theme with
the error of theme_parent_invalid.
 * Call send_origin_headers() earlier, to allow wp_die( '0' ) to properly
be returned in a domain mapping situation.
 * Fix the 'Save & Activate' message on themes.php.

fixes #20921.



git-svn-id: http://core.svn.wordpress.org/trunk@21069 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/customize.php

@@ -161,7 +161,7 @@ do_action( 'customize_controls_print_scripts' );
 		'url'      => array(
 			'preview'       => esc_url( $url ? $url : home_url( '/' ) ),
 			'parent'        => esc_url( admin_url() ),
-			'activated'     => esc_url( admin_url( 'themes.php?activated=true' ) ),
+			'activated'     => admin_url( 'themes.php?activated=true&previewed' ),
 			'ajax'          => esc_url( admin_url( 'admin-ajax.php', 'relative' ) ),
 			'allowed'       => array_map( 'esc_url', $allowed_urls ),
 			'isCrossDomain' => $cross_domain,