Add a "not_found" value in OriginVisitStatus.status

The idea is to be able to explicitly say for a visit to have not been able to find the origin URL. This is a specific type of loading error that will be needed for better loading scheduling.

The definition of "not found" being the responsibility of the loader (may be a DNS error, getting a 404, etc)

Note: Migration of the previous data in the archive is out-of-scope, that won't be done (at all).