Bugfix: Do not sync when `unsyncedfolders` file cannot be read

owncloudcmd now checks if the file specified by --unsyncedfolders exists
and can be read, before starting the sync. If it does not exist, show an
error message and quit immediately.
    
https://github.com/owncloud/client/issues/9165
https://github.com/owncloud/client/pull/9241
