Improve missing options error to be clearer in GPT Data Generator

Currently GPT Data Generator will check if there are any missing (nil) options it requires. However this message is currently vague:

./bin/generate-gpt-data:93:in `<main>': Some generate-gpt-data option(s) are not defined. Exiting... (RuntimeError)

We should improve this message to call out the missing option(s) specifically for better guidance to the user.