QModbusReply seems to never finish in some rare cases:
For instance if a request is sent and before the reply arrives, the
modbus adapter is unplugged. This change should keep upper logic
working which relies on requests finishing eventually.
Also fixes the issue that we emit finished() multiple times if
QModbusClient retries multiple times and emits error() for each
attempt. We're only interested in a single result anyways.
