Improve error reporting and robustness for VPN process handling.

- Add detailed error dialogs for abnormal exits and QProcess errors (e.g., failed sudo, crashed process).
- Ensure unknown VPN profile requests do not crash the GUI.
- Introduce delayed error reporting to avoid duplicate dialogs when vpnLogger generates specific messages.
- Add safeguards to stopVPN to prevent normal stops from being flagged as errors.
- Expand test cases (`90_gui.sh`) to validate error handling and GUI resilience.