storage*: Do not allow unknown visit status in origin_visit*_get_latest
Without this, that made some storage (pg-storage) fail.
As a future plan, make the VISIT_STATUSES an enum (and also most probably make the OriginVisitStatus.status field an enum as well).
That touches too many clients to change it right now though. I'll do it later.
This also fixes hidden problems on tests which used the in-memory storage (algos.test_origins).
Test Plan
tox
Migrated from D3680 (view on Phabricator)