Improve error handling and reporting for failed VPN connection attempts (issue #164):

- Add detailed error messages for various failure scenarios (e.g., unresolved gateway, authentication errors, timed out interfaces).
- Introduce robust detection and reporting of connection failures, including silent exits and conflicting VPN routes.
- Enhance `vpnLogger` and `vpnManager` with logic to distinguish user-initiated stops from failed attempts.
- Expand tests in `90_gui.sh` to validate error dialogs and ensure redundant reports are prevented.