Add Fallback Handling When Repository URL Is Not Found

[naheel0]

Description

Currently, when a user pastes a repository URL into ReadmeGenAI and the repository does not exist (invalid URL, typo, deleted repo, or private repo without access), the system does not provide a clear fallback response.

We need to implement proper error handling and display a user-friendly fallback message when the repository cannot be found.

Expected Behavior

• If the provided GitHub repository URL:
• Does not exist
• Is misspelled
• Returns 404 from GitHub API
• Is private and inaccessible
• Has API rate limit issues
• The system should:
• Detect the failed API response.
• Stop README generation.
• Display a clean fallback UI message such as:
  Repository not found.
  Please check the URL and try again.

Example Scenario

Input:
https://github.com/user/non-existing-repo

• Current Result:
  Error / no proper feedback
• Expected Result:
  Friendly fallback message displayed to the user.