Audit: Does not allow abortion with ExitCode::Ok

[jennijuju]

brought by @anorth from the internal audit, that FVM ExitCode enumeration might be inappropriate for use in actors.

After initial investigation, we may confirm FVM DOES guarantee that aborts cannot be done with Ok exit code. ⇒ overrides to SysErrIllegalExitCode. Then the API that builtin-actors uses should not allow actors to abort with ExitCode::Ok - this might be included in the ExitCode refactor already, this ticket is to confirm that's the right assumption.

[anorth]

Duplicate of #144. Fixed by #191 and will improve further with #214