Remove use of system exit codes

[anorth]

Actors should not attempt to abort with a system exit code (values < 16). This is currently permitted by the VM only to match behaviour of the Go runtime (which drew the VM/actor boundary higher up). Remove all references to the system exit codes from the actors, using actor-level ones instead. See https://github.com/filecoin-project/fvm-specs/blob/main/07-errors.md

This will unblock filecoin-project/ref-fvm#253

[anorth]

Fixed by #191 and will improve further with #214

cc @dignifiedquire