core.loader: Allow vcs loaders to deal with not_found status
This will allow git, mercurial or svn loader implementations to mark the visit status as not_found when such event occurs (e.g failing to to remotely communicate with the remote server, archive not found, etc...).
Technically, this opens a means to trap a NotFound exception from the main loop. Which finalize the visit and mark its status as "not_found".
Depends on !173 (closed) Related to #3030
Migrated from D5039 (view on Phabricator)