code: func (g *Cloud) getRegionalDiskByName(diskName string) (*Disk, error) {
  	disk, err := g.findRegionalDiskByName(diskName)
  	if disk == nil && err == nil {
  		return nil, fmt.Errorf("GCE regional persistent disk not found: diskName=%q", diskName)
  	}
  	return disk, err
  }
docs: // Like findRegionalDiskByName, but returns an error if the disk is not found