Changes to debug simulator (no effect on scripts).

Changed the formatting of the error message on WaitUntilAtTarget() so
that it is easier to see why the manipulandum is not detected to be at
the target. This should have no effect on the scripts, as it is local
to the simulator.
Make mouse tracker to work for horizontal targets.

The mouse tracker simulator now places the manipulandum just to the
right of the box, not to the right of the target bar. This works for
the horizontal targets and since we never ask subjects to point to the
vertical targets when in the left position, it should work fine there,
too.