KeyError on private RTCM3 messages

[semuadmin]

Reported by @wdc-rsat in PyGPSClient repo:

pygpsclient 1.3.21 throws KeyError exceptions on receiving private STMicroelectronics and SwiftNav RTCM3 messages:

`KeyError: '999' Exception in Tkinter callback Traceback (most recent call last): ... if self._labelmsm and "MSM" in RTCM_MSGIDS[self.identity]:

KeyError: '4062'
Exception in Tkinter callback
Traceback (most recent call last):
...
if self._labelmsm and "MSM" in RTCM_MSGIDS[self.identity]:
~~~~~~~~~~~^^^^^^^^^^^^^^^
`