Improve fsm error code handling in psm

Improve fsm error code handling in psm
- Move the fsm execution to separate helper function, returning
  the most relevant exit code: if fsmSetup() failed then teardown
  return doesn't matter, but fsmTeardown() can report errors
  from missing hardlinks which we haven't checked until now.
- This also fixes enum vs int abuse on the exit code: fsm return
  codes are cpio error codes whereas psm codes are simple ok/fails.