Change the type of isaModifyJumpTarget

[travitch]

Currently, neither implementation ever returns Nothing and just panics if a non-jump is passed in or if the required jump is not realizable. There is a redundant check at the call sites if we want it to be able to just panic.

We can either make it "total" (modulo the panics) or just return 'Nothing' when we currently panic and push the panic to the caller. There is additional arch-specific information available to provide more accurate panics in the backend implementations, so that might be a useful concern. On the other hand, the call site (mapJumpAddress) is in a context that could turn the failure into an exception that could be recoverable by a renovate client.