Improve error messages and troubleshooting

Make error messages more helpful and actionable.

Requirements:

• Detailed error codes with links to documentation
• Suggest fixes for common errors
• Export detailed error log (JSON format)
• Integration with --debug flag

Example:

Error: Authentication failed (HTTP 401)
Possible causes:
  1. Invalid credentials
  2. OAuth token expired
  3. Insufficient permissions

Troubleshooting steps:
  - Verify credentials in .env file
  - Regenerate OAuth refresh token
  - Check API permissions in Google Cloud Console

For more help: https://docs.example.com/errors/AUTH_401

Estimated Effort: Small (2 days)

Related to: DAV Migration CLI - Phase 2