Separate error message for when GitHub username is invalid

[Harsh-Modi278]

Currently, regardless of the error in node server, "No roast available." error is show on UX. When the input username is invalid, error message like "Invalid username, please try again with valid username" can be more helpful.

[Haimantika]

The no roast available error is because the OpenAI API limit has been exceeded. I will update it on Monday.

[Harsh-Modi278]

I meant to have a separate error message for this error path: