Fix match error cause sync shutdown

When sun sync:go(), will shutdown the application sync, errors
>>=SUPERVISOR REPORT==== 10-Oct-2013::13:25:44 ===
>>     Supervisor: {local,sync}
>>    Context:    child_terminated
>>     Reason:     {{badmatch,undefined},
>>                  [{sync_scanner,'-handle_cast/2-fun-0-',2,
>>                               [{file,"src/sync_scanner.erl"},{line,147}]}

...

>>=INFO REPORT==== 10-Oct-2013::13:25:44 ===
>>   application: sync
>>    exited: shutdown
>>    type: temporary

The reason is sync_util:128 can't match sync_util:104, will throw a exception. 
we just modify AttempDir to {ok, AttemptDir} will resovle this problem, the application 
will run correctly. thank you for your reading.