Curation: use previous version of extract when new one creates a problem

[tidoust]

Once in a while, a curated extract may turn out to be invalid. This prevents the curation from succeeding. In turn, that prevents the publication of new npm packages of Webref (it does not necessarily prevent the release of the current package, if there's one, but it prevents the creation of a new one, at least)

That may happen when:

1. A new spec is added, and it's somewhat common to have invalid CSS/ IDL in such specs
2. A spec gets updated
3. A patch no longer applies

We may want to intervene in 3., but it would be great to switch to a mechanism where the curation falls back to the previous valid known state for the problematic extract and rather creates an issue to alert us that something needs fixing, so that releases with other updates may continue while this gets fixed.

I note it may be hard to fallback automatically when the test that fails is on the merged IDL, or the merged events file.