package.loader: Fail loading early when failing to create an origin/visit

Fix [1] unbound value error when failure to create either origin or origin-visit.

• [1] https://sentry.softwareheritage.org/share/issue/6c4750b56a114817bb088ec6e4965faf/

Test Plan

tox

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Migrated from D2495 (view on Phabricator)