Add option to disable offline error

[pbhogan]

I run into the offline error popup constantly as I often have to use my laptop in an environment where I don't have an internet connection. It's really frustrating to have to dismiss it every time I open Atom.

Honestly, it seems like this error should be disabled by default. Syncing settings is the kind of thing where I expect it to only bother me when it really needs to. No-one needs it to complain that you're offline on startup. Maybe when you manually trigger the sync from the menu I could see it making sense to do so.

Please silence the error for auto-sync on startup, or at least add an option to do so.