X11: Handles connection failure in forwarder
X11: Handles connection failure in forwarder

Before, if the forwarder couldn't connect a traceback would show as the
_forward thread died.

ViDA-NYU/reprozip,ViDA-NYU/reprozip,ViDA-NYU/reprozip,ViDA-NYU/reprozip,ViDA-NYU/reprozip,JosuaKrause/reprozip,JosuaKrause/reprozip,JosuaKrause/reprozip,JosuaKrause/reprozip