This outputs a proper message if the requirement is not met, instead of an obscure PHP error further in the test. These tests rely on multiple resized copies of a test JPEG image being generated and available. Follow-up to [49010], [49024], [49025]. See #50639, #50640. git-svn-id: https://develop.svn.wordpress.org/trunk@49045 602fd350-edb4-49c9-b593-d223f7449a82