sanity check on frequency field rnode

[yiids]

When one inputs a frequency that is obviously wrong, like 868100000 MHZ, sideband should warn you that this is impossible.
Now it quietly lets you do that, and without any indication what is going wrong, the rnode just does not get initialized.

[RFnexus]

It would be nice if there was an option for interfaces to fail gracefully on startup. For example any validation or configuration errors would just disable the interface and store the key and it's error. It would also be nice if this was accessible through the API so any frontend application could show the user where the errors are.

On a somewhat related note there is a discrepancy between the interfaces RNodeInterface and SerialInterface

With the RNodeInterface, if the serial port you defined is unavailable when you start RNS it will continue and try to bring up the interface later.

But with the SerialInterface, KISSInterface, and AX25KISSInterface if the serial port is unavailable RNS will fail and not start until the device becomes available.