loader.svn: Mark visit status as not_found when relevant

When failing to communicate with the svn server at initialization time, a visit status ends up in "not_found" status.

Other failures to communicate ends up in a visit status marked as "failed".

Related to swh-loader-core#3030

Test Plan

tox (failing until swh.loader.core > 0.16 it released)

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Migrated from D5046 (view on Phabricator)