PVF preparation: do not conflate errors (#6384)

* PVF preparation: do not conflate errors

+ Adds some more granularity to the prepare errors.
+ Better distinguish whether errors occur on the host side or the worker.
+ Do not kill the worker if the error happened on the host side.
+ Do not retry preparation if the error was `Panic`.
+ Removes unnecessary indirection with `Selected` type.

* Add missing docs, resolve TODOs

* Address review comments and remove TODOs

* Fix error in CI

* Undo unnecessary change

* Update couple of comments

* Don't return error for stream shutdown

* Update node/core/pvf/src/worker_common.rs