Handle bad embed

*This is not a bugfix, it's a feature.*

On medium.com if we type a bad embed data, it's converted in a normal paragraph.
So I've handle it the same way.

If we fail to retrieve content for an embed, instead of showing an ugly message to the user,
we just convert the content into a normal paragraph.

The error will be displayed in the console. I don't know if we need to keep this.

I haven't the time to add a test but I can check that next week if it's needed.