Better error message when GitHub REST API fails

[lionello]

Description

When GitHub API gets throttled the CLI just prints Error: 403 Forbidden, without much context. This PR prints the actual error message (if available) for better diagnosis.

Checklist

• I have performed a self-review of my code
• I have added appropriate tests
• I have updated the Defang CLI docs and/or README to reflect my changes, if necessary

[jordanstephens]

If you want to commit this as-is, that's OK, but I think we could push this a little further and try reading the rate limit headers and comparing them. Let me know if you want me to take a swing at it.