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.

(cherry picked from commit 9b6aa9e)
(cherry picked from commit 0e5fc0a29c4a98f781d4d4911b5f589f31c9f10e)
(cherry picked from commit 7dfa163cecb3e79daf35ad9858d0e9b725cb2234)
(cherry picked from commit 87aea85)

Author: yuwata

src/core/transaction.c

@@ -984,9 +984,9 @@ int transaction_add_job_and_dependencies(
                 if (manager_unit_cache_should_retry_load(unit)) {
                         assert(unit->load_state == UNIT_NOT_FOUND);
                         unit->load_state = UNIT_STUB;
-                        r = unit_load(unit);
-                        if (r < 0 || unit->load_state == UNIT_STUB)
-                                unit->load_state = UNIT_NOT_FOUND;
+                        unit->load_error = 0;
+                        (void) unit_load(unit);
+                        assert(unit->load_state != UNIT_STUB);
                 }
 
                 r = bus_unit_validate_load_state(unit, e);