delegated balance RPC fails with uninformative error when delegate is not registered
results in:
[ { "kind": "temporary", "id": "proto.003-PsddFKi3.tez.subtraction_underflow", "amounts": [ "0", "188607" ] } ]
The protocol code in delegate_storage.ml seems to be attempting to subtract the account's balance from the staking balance to obtain the delegated balance, but as the staking balance is 0 it fails. It should probably report either 0, an actual negative balance, or an error indicating that the contract is not a registered delegate.