FS-11444: Capture return code from `os.execute()`; stop attempting to concat boolean/nil to error string.

os.execute() returns [3
variables](https://github.com/lua/lua/blob/f59e6a93c0ad38a27a420e51abf8f13d962446b5/lauxlib.c#L286).
Capture the 2nd (e.g. "exit") and 3rd (e.g. 0) in addition to the first
which would be merely `true` on success or `nil` on error.