GUI: Popup error message box when ASSERT fails

Currently, a failed ASSERT results in a silent crash, or at most a terminal message like

WARNING: Qt has caught an exception thrown from an event handler. Throwing
exceptions from an event handler is not supported in Qt.
You must not let any exception whatsoever propagate through Qt code.
If that is not possible, in Qt 5 you must at least reimplement
QCoreApplication::notify() and catch all exceptions there.

Segmentation fault