core/transaction: do not override unit load state when unit_load() failed

core/transaction: do not override unit load state when unit_load() failed

When unit_load() failed for some reasons, previously we overrided the
load state with UNIT_NOT_FOUND, but we did not update the
Unit.fragment_not_found_timestamp_hash. So, the unit may be loaded
multiple times when the unit is in a dependency list of another unit,
as manager_unit_cache_should_retry_load() will be true again even on
next call.
Let's not override the unit state set by unit_load().

Note, after unit_load(), the unit state should not be UNIT_STUB.
Let's also add the assertion about that.

This change is important when combined with the next commit, as with the
next commit we will restart the FOREACH_UNIT_DEPENDENCY() loop if an unit
is reloaded, hence overriding load state with UNIT_NOT_FOUND may cause
infinit loop.