{Cnt|Dir}Loader: Fix standard/nar hash mismatch behavior to fail loading

Prior to this commit, there was a discrepancy between the hash mismatch computations
with "standard" and "nar" computations. This commit fixes the gap between those.

When a hash mismatch occurs, either "nar" or "standard", the issue is caught and the
next mirror url is checked. At the end of it all, if nothing is loaded and errors
exist, this is raised. This fails the visit.

This also adds the missing tests.

Related to T3781