Looks up an artifact with given name from the database.

def ReadArtifact(self, name, cursor=None):
    """Looks up an artifact with given name from the database."""
    cursor.execute("SELECT definition FROM artifacts WHERE name = %s", [name])

    row = cursor.fetchone()
    if row is None:
      raise db.UnknownArtifactError(name)
    else:
      return _RowToArtifact(row)