Header images: Handle cropping failures.

props ehg.
see #21785.

Built from https://develop.svn.wordpress.org/trunk@27946


git-svn-id: http://core.svn.wordpress.org/trunk@27776 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Header images: Accessibility and style improvements. Headers no longer jump around when chosen.

props mcsf, ehg.
see #21785.

Built from https://develop.svn.wordpress.org/trunk@27947


git-svn-id: http://core.svn.wordpress.org/trunk@27777 1a063a9b-81f0-0310-95a4-ce76da25c4cd