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'
```