BGE: fix crash and return boolean on scene.replace()

Scene replacement with invalid scene name was crashing blender,
now it's a no-op.
KS_Scene.replace() to return a boolean to indicate if the scene
is valid and is scheduled for replacement. This allows more
robust game management.

doc/python_api/rst/bge_types/bge.types.KX_Scene.rst

@@ -152,6 +152,8 @@ base class --- :class:`PyObjectPlus`
 
       :arg scene: The name of the scene to replace this scene with.
       :type scene: string
+      :return: True if the scene exists and was scheduled for addition, False otherwise.
+      :rtype: boolean
 
    .. method:: suspend()