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 that already have a scope established.

• 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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

n-api