Add offline fetch of repository error to view model errors signal

[stevemoser]

Not sure if this is the right way to fix the offline error but it fixes #16 for me. Also it seems the the error alert view eventually appears but not instantly, probably because [alert show] isn't on the main thread. :beers:

[jspahrsummers]

Thanks! Just a couple notes.

[stevemoser]

Np. I think I fixed the white space, let me know otherwise. Not sure as I never use tabs with Xcode.

[jspahrsummers]

Yep, the indentation is certainly fixed, but there was a note about the threading too (regarding self->_errors sendNext:):

This should only happen on the main thread. You can use [RACScheduler.mainThreadScheduler schedule:] to do that.