support opening files of unsync repo from activity tab

![screen shot 2014-11-26 at 14 37 07](https://cloud.githubusercontent.com/assets/2123461/5197235/cd71f988-7579-11e4-9375-e0536015aac2.png)

Yeah, ``Failed to download file XXX`` is kind of better. What about this syntax https://discussions.apple.com/thread/6540292?start=60&tstart=0 ? it seems it is used more commonly.