`Onebox.preview` can return 0-to-n errors, where the errors are missing OpenGraph attributes (e.g. title, description, image, etc.). If any of these attributes are missing, we construct an error message and attach it to the Oneboxer preview HTML. The error message is something like:
“Sorry, we were unable to generate a preview for this web page, because the following oEmbed / OpenGraph tags could not be found: description, image”
However, if the only missing tag is `image` we don’t need to display the error, as we have enough other data (title, description, etc.) to construct a useful/complete Onebox.
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