loader.git: Mark visit status as not_found when relevant
When the initial communication with the git server is failing initially (e.g repository is not found), this marks the visit status as not_found.
When the initial communication is ok but a failure occurs during the fetch step (e.g pack file too big, ...), the visit status is marked as failed.
Related to swh-loader-core#3030
Test Plan
tox (failing until swh.loader.core > 0.16)
Migrated from D5041 (view on Phabricator)