Error on unknown message id.

```
$ pingproxy.py --device /dev/serial/ping/Ping1D-id-1-t-1-m-1-v-3.26
error unpacking payload: 6
msg_data: BR�P
, header: (66, 82, 2, 6, 0, 0)
Traceback (most recent call last):
File "/usr/local/bin/pingproxy.py", line 112, in
proxy.run()
File "/usr/local/bin/pingproxy.py", line 76, in run
if e.errno == errno.EAGAIN:
AttributeError: 'exceptions.KeyError' object has no attribute 'errno'
```

Start with pingproxy.py around lines 76 and 112, then trace how an unknown message id reaches the payload-unpacking and exception-handling paths. Reproduce the reported command if the Ping1D device or equivalent input is available. Done means unknown message data is handled without the shown AttributeError and the proxy continues or reports the original error clearly.

由索引模型根据 Issue 内容生成。