Introduce WP_Theme->exists() to check if the queried theme actually exists....

Introduce WP_Theme->exists() to check if the queried theme actually exists. WP_Theme->exists() is a subset of errors() -- a theme with errors may still exist, but a theme that does not exist has an error of theme_not_found. wp_get_theme() now returns false if the theme does not exist. Improve scandir() and get_files() logic. see #20103.

git-svn-id: https://core.svn.wordpress.org/trunk@20312 1a063a9b-81f0-0310-95a4-ce76da25c4cd