Embeds: Fix Error Display when Embed Fails

[gwwar]

When attempting to embed an unsupported URL, we show an empty block instead of the embed URL block with a human readable error.

To reproduce try embedding a Medium post.

Expected	Actual

[carinapilar]

When trying to embed a video from the FramaTube platform (with the video's URL), it shows the following:

It's just a video, and it loads on the live site, the messaging sounds confusing.

[lancewillett]

From my experience, the fallback should be a plain URL, the same result as "Convert to a link."

[github-actions]

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