C++: Catch msgpack exceptions and throw our exception

Before this, listening to malformed Python heartbeats:

💥

Now:

|2024-05-17 19:33:53.574|  WARNING [CHP] Error decoding message: malformed data

This is a step towards fixing #26

I took the freedom to adjust the Python implementation to be more protocol-conform (all message parts are MsgPack packed), @hanno.perrey:

The heartbeat and extrasystole message frame MUST be encoded according to the MessagePack specification. It SHALL contain two strings, followed by a 64-bit timestamp, a 1-OCTET integer value and a 2-OCTET integer value.

(via https://constellation.pages.desy.de/protocols/chp.html#message-content)