This error type my be thrown when registering custom descriptors

This is equivilent to the default tank.descriptor.io_descriptor init
method.

Both functions now pass in the baseclass to register against.

The hook has changed it's method from ``register_types`` to ``register_io_descriptors``.

The base class of this hook will run the same shotgun logic as before by calling the method in the __init__ of ``tank.descriptor.io_descriptor``.

``_register_io_descriptor_hook`` to call the RegisterDescriptors hook ``register_io_descriptors`` via the DescriptorManager method.