Need Descriptive Error Message when generational_ea() is called without an ops.pool() operator

[escott-mitre]

Problem

Issue encountered by @anmol-srivastava-mitre:

Forget to pool, and you are met with a crazy oscure error message about the secret elitist_survival operator that lives inside generational_ea

MWE

ea = generational_ea(
        max_generations=100,
        pop_size=10,
                         
        problem=my_problem,
                         
        representation=my_representation,

        pipeline=[
            ops.tournament_selection,
            ops.clone,
            #
            #
            ops.evaluate
        ]
    )

Expected Behavior

A nice error message to the effect that "Your operator pipeline outputted a generator when it was done, but generational_ea expects to receive the new population as a list. Perhaps you forgot an ops.pool() operator?"

Actual Behavior

ValueError: Operator <function elitist_survival at 0x0000029754CF1900> received a <class 'generator'> as input, but expected a list.