Always set WP_Theme->template even when there is an error and we have no idea...

Always set WP_Theme->template even when there is an error and we have no idea what the template is. (Assume it is the stylesheet.) Prevents a number of issues including WP_Theme->is_child_theme() lying. Tidy the theme editor for broken themes and themes with no templates (PHP files), or no template (parent), or are broken. Allow broken themes to be edited. see #20103.

git-svn-id: https://core.svn.wordpress.org/trunk@20315 1a063a9b-81f0-0310-95a4-ce76da25c4cd