Don't allow the plugin to function if the root dir is unreadable.

If the path returned by `HMBKP_Scheduled_Backup::get_root()` isn't readable then we should disable the plugin in the same way we disable the plugin if `HMBKP_Scheduled_Backup::get_path()` isn't writable.

This PR adds a check to `hmbkp_possible()` for if the root path is readable and if it isn't a warning message is shown and the plugin functionality is disabled.