Code review comment for lp:~wgrant/launchpad/bulk-insert

Change the assert on line 82. It should be explicitly raised if the method has an integrity issue. Is this an AssertionError? I read "The Storm columns to insert values into. Must be from a single class", which implies this is a ValueError. Maybe?
    if len(clses) == 1:
        raise ValueError('The Storm columns to insert values into must be from a single class.')