Improve error message for chip -> chip learning

[arvoelke]

import nengo
import nengo_loihi

with nengo.Network() as model:
    nengo_loihi.add_params(model)
    
    a = nengo.Ensemble(100, 1)
    b = nengo.Ensemble(100, 1)
    model.config[b].on_chip = True
    
    nengo.Connection(a, b, learning_rule_type=nengo.PES())

with nengo_loihi.Simulator(model) as sim:
    pass

~/CTN/nengo-loihi/nengo_loihi/splitter.py in split(net, precompute, node_neurons, node_tau, remove_passthrough)
    132 
    133     # --- Step 2: place simple connections
--> 134     place_internetwork_connections(networks, conns)
    135 
    136     # --- Step 3: split complex connections

~/CTN/nengo-loihi/nengo_loihi/splitter.py in place_internetwork_connections(networks, conns)
    234         if pre_loc == post_loc:
    235             if pre_loc == "chip":
--> 236                 assert conn.learning_rule_type is None
    237             networks.move(conn, pre_loc)
    238 

AssertionError: