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)
Migrated from D5046 (view on Phabricator)