In FFI, don't panic if visit_scan_files returns an error

[nicklan]

This isn't actually quite as easy at it seems. To become an ExternResult you need an engine, and as the APIs are now you likely won't have an engine when calling this, because kernel_scan_data_next doesn't have one, so it can't pass it in when it calls this function.

We could take an engine arg in kernel_scan_data_next and thread it all the way through. This might be the right thing to do. We could also just require an engine in visit_scan_data and trust that engines can either shove it in the context or have global access to an engine.