Provide a more useful error message on reuse of destroyed layer.

It's already hard to tell what's going on because the stack trace you see is
the code that tries to use the layer, which often doesn't tell you anything
useful about who created the layer (particularly if the layer add is part of an
animation which is often how this sort of failure comes about).

Having the toString of the layer won't necessarily obviously solve this
problem, but it's better than nothing.