Add `error_call` to backend-ish functions

`pack()`, `chop()`, `unpack()`, and `unchop()`

They have proven to mainly be useful as helpers for implementing `unnest()` and `separate_wider_regex()` and it would be nice to show `unnest()` in the error call when the failure actually happens in one of these lower level functions

Default to `error_call = current_env()` for the people who do call these directly.