Header images: Handle cropping failures.

props ehg.
see #21785.


git-svn-id: https://develop.svn.wordpress.org/trunk@27946 602fd350-edb4-49c9-b593-d223f7449a82
Header images: Accessibility and style improvements. Headers no longer jump around when chosen.

props mcsf, ehg.
see #21785.


git-svn-id: https://develop.svn.wordpress.org/trunk@27947 602fd350-edb4-49c9-b593-d223f7449a82