package: Mark visit as not_found when relevant
If any errors happen during the communication with the origin to retrieve package information, the visit fails and its status is marked as not_found.
Only pypi, npm and nixguix package loaders are impacted. The other loaders do not read anything from their url so there is no way to trigger such possibility.
Note that the nixguix loader got refactored to avoid the side-effect of reading data out of the url within the constructor. It was necessary so the check fails and the visit status is dealt with as described. Also, this unifies it with how pypi and npm loaders deals with the url communication.
Related to #3030 Depends on !172 (closed)
Migrated from D5035 (view on Phabricator)