code: func (m *model) checkFolderRunningLocked(folder string) error {
  	_, ok := m.folderRunners[folder]
  	if ok {
  		return nil
  	}
  
  	if cfg, ok := m.cfg.Folder(folder); !ok {
  		return errFolderMissing
  	} else if cfg.Paused {
  		return ErrFolderPaused
  	}
  
  	return errFolderNotRunning
  }
docs: // checkFolderRunningLocked returns nil if the folder is up and running and a
  // descriptive error if not.
  // Need to hold (read) lock on m.fmut when calling this.