Returned by tryFetchRichContent to GET /api/remote/preview, which reports
ok as reached, and to refreshDumpMetadata, which gives up when it is
false.
ok: boolean
False when the page couldn't be reached at all, as opposed to reached and found to carry no metadata. A caller that already holds metadata must not overwrite it with the failure stub.
optional
content: RichContent
On failure, a stub holding the URL and its host name. Missing only when the URL doesn't parse.