core: log unit failure with type-specific result code

This slightly changes how we log about failures. Previously,
service_enter_dead() would log that a service unit failed along with its
result code, and unit_notify() would do this again but without the
result code. For other unit types only the latter would take effect.

This cleans this up: we keep the message in unit_notify() only for debug
purposes, and add type-specific log lines to all our unit types that can
fail, and always place them before unit_notify() is invoked.

Or in other words: the duplicate log message for service units is
removed, and all other unit types get a more useful line with the
precise result code.

src/core/scope.c

@@ -253,6 +253,9 @@ static void scope_enter_dead(Scope *s, ScopeResult f) {
         if (s->result == SCOPE_SUCCESS)
                 s->result = f;
 
+        if (s->result != SCOPE_SUCCESS)
+                log_unit_warning(UNIT(s), "Failed with result '%s'.", scope_result_to_string(s->result));
+
         scope_set_state(s, s->result != SCOPE_SUCCESS ? SCOPE_FAILED : SCOPE_DEAD);
 }