WP_Theme: Return false from the display() method when get() fails.

Fixes a potential warning on the themes page when the active theme is deleted.

fixes #26873.

Built from https://develop.svn.wordpress.org/trunk@27745


git-svn-id: http://core.svn.wordpress.org/trunk@27582 1a063a9b-81f0-0310-95a4-ce76da25c4cd