Overview:   [VC]Handle a exception, add some log

Module:     VeryCD Mirror
Platform:   Offline
Bug ID:     None
Description:Handle the url error in crawler which shutdown the thread. Each time
            the error happened the content of the db is "Error 0".
            Add some log to watch the thread status.
TODO List:
            1.Repull the items whose content is "Error 0"
            2.Update command's names just like git commands