Avoid inconsistent state when remove fails.

            final StringBuilder command = new StringBuilder();

            command.append("bandwidth removeiquota ").append(iface);

            mActiveQuotaIfaces.remove(iface);

            mActiveAlertIfaces.remove(iface);

            try {

                // TODO: support quota shared across interfaces

                mConnector.doCommand(command.toString());

                mActiveQuotaIfaces.remove(iface);

                mActiveAlertIfaces.remove(iface);

            } catch (NativeDaemonConnectorException e) {

                // TODO: include current iptables state

                throw new IllegalStateException("Error communicating to native daemon", e);

            }

        }