n-api: add missing exception checking

Add checks for a pending exception in napi_make_callback
after the callback has been invoked.  If there is a pending
exception then we need to avoid checking the result as that
will not be able to complete properly.

Add additional checks to the unit test for napi_make_callback
to catch this case.

PR-URL: https://github.com/nodejs/node/pull/19362
Fixes: https://github.com/nodejs/node-addon-api/issues/235


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>