Code generator should log a diagnostic when attempting to create linking code for private methods

[McJones]

Currently if you have the following code:

[YarnCommand("thing")]
private void DoAThing()
{
    Debug.Log("doing a thing");
}

The automatic code generator that links this runs without complaint but later at runtime the game will report that it can't find the <<thing>> command.
The code gen should log a diagnostic that the methods access prevents it from working.