Potential fix for code scanning alert no. 2: Information exposure through an exception

[GitTimeraider]

Potential fix for https://github.com/GitTimeraider/GithubBackup-docker/security/code-scanning/2

In general, sensitive exception information should only be logged on the server, not returned to users. To fix this:

• Change the code to log the exception details (including the message and stack trace) using the server's logging facility.
• Only return a generic error message to the user, such as "An internal error occurred during backup".

Detailed Steps:

• In test_backup_with_context, replace the return statement that exposes str(e) to instead return a generic error string.
• Make sure you log the full exception (possibly with stack trace) using Python's logging module, so the details are captured for debugging.
• The calling code at line 541 will remain unchanged, resulting in only the generic message being sent in the JSON response.

All required functionality (logging, Flask, etc.) is already imported and available.

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[Copilot]

Pull Request Overview

This PR addresses a security vulnerability related to information exposure through exceptions by replacing detailed error messages returned to users with generic messages while preserving detailed logging for debugging purposes.

• Enhanced exception logging to include stack traces using exc_info=True
• Replaced user-facing error message containing exception details with a generic message

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