qt: terminate the application on scene graph error in CompositorDirectComposition

When scene graph encounters an error, we are already past the latest stage where recovery is possible. The interface remains nonfunctional in this case, thus the application must be terminated. This is already done by Qt itself when the signal is not connected, but in this case we connect to the signal to let the waiting video thread continue and to print the error.

Note that "If a signal is connected to several slots, the slots are activated in the same order in which the connections were made". So, in this case exit is going to be called after the error is printed, which is the intended behavior.

Since !6013 was not merged, we need to do this otherwise the application may end up with broken interface.

Request review @chub.