Cannot recover from validation errors when syncing from Dropbox

[lyahdav]

We have a scenario where invalid data exists (e.g. a required field is nil) on the Dropbox datastore and then it comes into our app. The sync fails because of Core Data validations. We would like to handle this error in our app, but currently there's no way to know something went wrong. We have a solution here:
vijayj@d89df6d. The notification that's posted when incoming changes are saved now includes an error if something went wrong.
I didn't make this a pull request because it's on top of a branch that we currently use in our app, but you should be able to cherry pick the commit over.