Improve loaders to deal with new visit status events

The model and db now supports the not_found and failed status. Update the loader implementations to reference such information.

Note: It's also the opportunity to make the loader reference the visit-status type when discussing with the storage [1]

This is a state diagram explaining the different statuses :

• [1] !170 (closed)

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Migrated from T3030 (view on Phabricator)