Minor Sandcastle example cleanup

Low-hanging fruit from #1685.

Lots more to come, of course.

This also changes `removeEventListener` to return a boolean indicating if the event was removed instead of throwing an exception.  This is consistent with `remove` on all our collections and makes the camera ICRF examples cleaner.
I know this was already there, but "an rectangle" is incorrect now.  Presumably this was "an extent" before the rename.