n-api: Handle fatal exception in async callback

 - Create a handle scope before invoking the async completion
   callback, because it is basically always needed, easy for user
   code to forget, and this makes it more consistent with ordinary
   N-API function callbacks.

 - Check for an unhandled JS exception after invoking an async
   completion callback, and report it via `node::FatalException()`.

 - Add a corresponding test case for an exception in async callback.

Previously, any unhandled JS exception thrown from a
`napi_async_complete_callback` would be silently ignored. Among other
things this meant assertions in some test cases could be undetected.

PR-URL: https://github.com/nodejs/node/pull/12838


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>