polkadot --version check for execute and prepare binary

[ggwpez]

This is more of Q and suggestion: maybe the version output of the polkadot binary should also do a light self-check to ensure that the execute and prepare worker binaries exist and have the expected version.

Someone in the validator lounge posted this which got me thinking:

I m now concerned about the installation (polkadot-execute-worker and polkadot-prepare-worker returns command not found whereas polkadot --version returns polkadot 1.16.2-dba2dd59101) ...