This unifies the logic of receiving an error when broadcasting a
confirmed transaction through btcd's/bitcoind's RPC interface. The btcd
dependency update is required in order for it to match bitcoind's
behavior. For older nodes that have yet to update, the confirmed
transaction will still be caught by the "transaction already exists"
case. This is not needed for bitcoind however, because its been sending
the same RPC error code for several major releases now.
