Cloud storage user deauthenticates their access token, or an access token expires

[crspybits]

This is a case related to removal of a user (e.g., see #78, #52), but different. The user would not delete themselves, rather they would go into their Google Drive or Dropbox and just deauthenticate their Oauth token(s). I think this would also occur for Google Drive cloud storage users as well if they were to not regenerate their refresh token after it expires-- e.g., if they were to stop using the SyncServer client app for a long period of time.

This would result in SyncServer being unable to access their cloud storage for any purpose (upload, download, deletion). The goal here should be to not have SyncServer (or a client app) fail because of this but rather to avoid the relevant files, at least temporarily. A question is: Do we have a way to distinguish between a cloud storage access failure for these reasons versus a failure because of some other problem (e.g., general purpose network problem)?