Report specs for which we only have fallback data in pre-release PR

[tidoust]

See #856 (comment)

The pre-release PR should alert us about the list of extracts of specs that could not be crawled, because these extracts could be outdated or could use an outdated JSON structure. That may trigger a bunch of temporary updates to the pre-release PRs ("server is down!", "server is up again!", "server is down again!") but we need to have the info when we review the PR.

NB: note we should not create a pre-release PR if the diff only contains extracts that could not be crawled.