Exec-next: improve null propagation errors

[rmosolgo]

Null propagation basically works, but the errors produced aren't quite right yet.

Fixes #5622

TODO:

• Implement top-level eager killing I removed it -- the existing support worked well enough.
• Further unify finalizers and error results? These are almost the exact same structure.
• Look at where finalizers and error results are merged. I think they clobber each other in some cases.