Migrate one Artifact from AFF4 to REL_DB.

def _MigrateArtifact(artifact):
  """Migrate one Artifact from AFF4 to REL_DB."""
  name = Text(artifact.name)

  try:
    logging.info("Migating %s", name)
    data_store.REL_DB.WriteArtifact(artifact)
    logging.info("  Wrote %s", name)
  except db.DuplicatedArtifactError:
    logging.info("  Skipped %s, because artifact already exists.", name)