Tighten up use of special ports

[bocchino]

Currently special ports for commands, telemetry, etc. are allowed even when there are no commands or telemetry channels defined. Possibly we should exclude this case, and throw an error when analyzing the model.

Related to #339.

[timcanham]

For what it's worth, I've been wanting FPP to fail if the F Prime special ports and their FPP features (commands/telemetry/events) are not matched, but I was waiting for the new autocoder as the starting point. Not having them can introduce confusing errors downstream. In the past, if I didn't have anything yet for a feature, I created placeholders just so I can hook up the topology early.