Worker returns action status

luigi.run() returns boolean value indicating whether everything was executed successfully (i.e. adding and running the tasks - which means that either given tasks and all of their dependencies were already done or completed during this particular execution).

Without this patch one doesn't know whether all tasks succeeded or failed (you can of course register callbacks on failing / broken / etc task, but those callbacks will be executed in fork processes, not in the process, that called luigi.run)