[Internal]: Handle GitHub API errors in release_notes.py

[jvstme]

This improves the script error message if the
GitHub API call is not successful (e.g., if the
token is expired).

Before:

TypeError: string indices must be integers, not 'str'

After:

Exception: Error getting GitHub releases; status: 401, body: {
  "message": "Bad credentials",
  "documentation_url": "https://docs.github.com/rest",
  "status": "401"
}