Widgets: Style the Image Widget's link field as invalid when a bad URL is supplied.

Fixes issue where user may supply an invalid URL and be unable to save the widget on the Widgets admin screen without being told why.

Amends [41252].
See #41274.
Fixes #42488.

Built from https://develop.svn.wordpress.org/trunk@42140


git-svn-id: https://core.svn.wordpress.org/trunk@41971 1a063a9b-81f0-0310-95a4-ce76da25c4cd