Download locally existing remotely modified files in on-demand mode

If a file doesn't exist locally it won't be downloaded until opened, but
if a file does exist locally and has been modified remotely the latest
version will be downloaded at sync time regardless of the file sync
mode. (If a locally existing file has been updated remotely, just
opening it won't currently download the latest version. You have to
sync. This may change in the future.)
Remove debug line and update error message
Fix update marking for uploaded file versions that exist remotely

If a file version existed on the server, the uploaded attachment wasn't
marked as updated until the next sync.