Checking PX4 messages at startup

[damien-robotsix]

Hello,

The compatibility of the PX4 messages are tested at startup but not their availability. I don't know to which extend it can be implemented but for now if one message if missing in the controller we get a throw when we ask for the last message for example with no previous warning. It would be nice to have a system to check that the required messages are really available on PX4 side at initialization.

If it exist already I doesn't work for vehicle_angular_velocity for example (did I miss something when I did the PR on this?)

Also the name of the topic in the throw would be a + :D

[bkueng]

There's no check yet. My preferred approach is to have a mechanism to request a message at a given update rate. And this could then also return an error if it's not available.