Ask user whether to init a new repo if adding existing repo fails

[LY871]

Description

When a user attempts to add an existing repository but the connection fails (e.g., the folder is not a valid Borg repository), the application now intercepts the error and prompts the user via a QMessageBox. This prompt offers the option to initialize a new repository at that location immediately, improving the onboarding flow for new users.

Related Issue

Closes #1799
#1799

Motivation and Context

Currently, if a user points Vorta to a non-initialized folder, they simply receive an error message. This change reduces friction by suggesting the logical next step

How Has This Been Tested?

Environment: WSL2 (Ubuntu 22.04) running Vorta in a Python 3.12 virtual environment.
Test Case 1: Attempted to add an empty directory. Verified that the "Repository Not Found" dialog appeared.
Test Case 2: Clicked "Yes" on the dialog. Verified that it successfully triggered the new_repo workflow.
Test Case 3: Clicked "No" on the dialog. Verified that the standard error status message was displayed.

Types of changes

• [x ] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ x] I have read the CONTRIBUTING guide.
• [ x] My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• [ x] All new and existing tests passed.

I provide my contribution under the terms of the license of this repository and I affirm the Developer Certificate of Origin.