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/timer.c

@@ -296,6 +296,9 @@ static void timer_enter_dead(Timer *t, TimerResult f) {
         if (t->result == TIMER_SUCCESS)
                 t->result = f;
 
+        if (t->result != TIMER_SUCCESS)
+                log_unit_warning(UNIT(t), "Failed with result '%s'.", timer_result_to_string(t->result));
+
         timer_set_state(t, t->result != TIMER_SUCCESS ? TIMER_FAILED : TIMER_DEAD);
 }