Widgets: Harden logic for checking for valid media URLs.

* Update deprecated `isHostedVideo` method to always return `true` since all oEmbeds are now supported.
* Disable `scanImage` for non-image media widgets.
* Ensure embed URL field element is matched from current media frame instance due to bug where media frames are not destroyed upon closing.
* Update error message for invalid video URLs to ask user to check the URL for validity.

Props gk.loveweb, octalmage, westonruter.
See #42039, #40935.
Fixes #41052.

Built from https://develop.svn.wordpress.org/trunk@41933


git-svn-id: https://core.svn.wordpress.org/trunk@41767 1a063a9b-81f0-0310-95a4-ce76da25c4cd