Write helpful error message if incompatible topic protocol versions try to communicate

[cstim]

The vrpc topic structure is occasionally being changed, last time e.g. between vrpc version 2.1.0-alpha.0 and 2.1.0-alpha.1 IIRC. This means that e.g. an Agent of version 2.1.0-alpha.0 and a server with Remote of version 2.1.1 cannot communicate with each other, even though both can correctly authenticate at the MQTT broker and no error is shown there, which makes the cause for the communication failure hard to detect. This ticket asks to introduce some extra checks on the server side whether an agent of an older protocol version tries to communicate here, to avoid these hard-to-detect situation.

Written as a user story: As a service technician who updates the server software instance to vrpc-2.1.1, but forgets to update the remote agent instance from 2.1.0-alpha.0 to some newer version, I want to see a helpful error message on either the server or the agent side stating that a connection attempt from another vrpc instance has been detected but is using an incompatible version of the vrpc protocol. The error message should preferrably be on the server side (new software), but could alternatively also appear in the agent (old software), as long as at least one message appears somewhere.