n-api: refactor a previous commit

This is a refactoring of https://github.com/nodejs/node/issues/27628
following https://github.com/nodejs/node/pull/28505.

This change factors out functions `add_last_status()` and
`add_returned_status()` so they may be reused in the tests for passing
information about the last error status and/or a returned `napi_status`
to JavaScript.

PR-URL: https://github.com/nodejs/node/pull/28848


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>