Secure embeds in the editor (first run):
- When the user pastes an embeddable http URL, try to get the https embed.
- If an embed provider doesn't support ssl embeds, show a placeholder/error message.
- Revise the way we return error messages.
See #28195, #28507.
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git-svn-id: http://core.svn.wordpress.org/trunk@28718 1a063a9b-81f0-0310-95a4-ce76da25c4cd