Better error message when an oEmbed preview cannot be shown in the editor

[azaozz]

The current message: "Preview not available. %s cannot be embedded securely." is not clear and may sound "scary" to some users.

[azaozz]

This is shown then the admin is served over ssl but the oEmbed doesn't support ssl. The users can still preview the post to see the embeds.

Perhaps only Preview not available. is enough?

[azaozz]

In 30591:

Better error message when the admin is ssl and non-ssl oEmbed previews are not available in the editor. Props Japh, fixes #30533.

[nacin]

Great change.