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".

Related to T3030