Resolve "Handle ONCat failures gracefully"

If ONCat fails to load or goes down after loading, this should allow the application to continue running smoothly and try again during the next sync opportunity (either 60 seconds or when the user changes IPTS at this point). I added some logic to the data previews to show an error message if ONCat didn't find the requested datafile rather than showing a spinner forever.

Also, I moved the periodic sync to the view model so that I could ensure that the view is updated after sync (noticed that it wasn't updating properly after reconnecting during a test).

Closes #27 (closed)