Fix #VALUE! error by adding API key validation and improved error handling

[Copilot]

Problem

Users who follow the installation instructions but don't replace the default API key placeholder (sk-YOUR-CHATGPT-KEY-HERE) with their actual OpenAI API key receive a cryptic #VALUE! error in Excel when trying to use any of the ChatGPT functions, as shown in the issue screenshot.

Solution

This PR adds several improvements to make the error more user-friendly and guide users to the correct solution:

1. Added validation check for the API key to detect if it's still the default placeholder
2. Added a clear, descriptive error message that explicitly tells users to replace the API key
3. Improved the API response parsing logic to be more robust and handle edge cases better
4. Updated documentation to emphasize the importance of replacing the API key
5. Added troubleshooting information about #VALUE! errors in the README

Changes:

• Added API key validation check in GetChatGPTResponse function
• Improved response parsing to avoid reliance on hardcoded offsets and to better handle API response variations
• Updated README to make the API key replacement step more prominent
• Added troubleshooting note specifically for #VALUE! errors
• Updated code comments to emphasize the API key requirement

These changes provide a much better user experience by giving clear, actionable feedback when users forget to replace the placeholder API key with their own.

Fixes #4.

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