Specify errors for add_free()

[EwanC]

Decide if command_graph<graph_state::modifiable>::add_free() show throw an exception under any of the following circumstances, or others not listed here.

• If data not allocated by add_malloc_device
• If data is already freed
• If data is not valid address

If so, update the spec to reflect that this behaviour.

[EwanC]

In sycl::free the wording is that that behaviour is not defined on invalid usage, so I think we should stick with that rather than specifying that an exception is thrown

I'll come up with a spec PR for adding wording about undefined behaviour on invalid usage.

[EwanC]

Closing this as not-done in favour of the more general issue - #39