More informative errors when a chip config cannot be made

[ssinhaleite]

When the DynapcnnNetwork.make_config fails, it is often not clear why. It would be helpful to get a brief error message of what failed.

Also, when no suitable mapping of layers to chips is found, currently a ValueError is raised, which is quite generic and makes it hard to catch this specific error in other methods. It might help to include a more specific Error type.