Classes of errors

Sync argument errors. These should "throw" and many do because that's how luaL_check* works.
Errors while async. These happened while trying to do an async action. They should either be handed to the callback or emitted as error.
Error while executing async callbacks. If an error handler has a bug, there isn't much we can do. Either "throw" or send as "error" event if possible.

Error handling

async errors are always handed to lua somewhere with the exception of "error" events that aren't listened for
uncaught "error" events and sync errors will crash the process and should always show a stack trace

I know errno and path, this is a fs error

luv_io_error(L, req->errorno, NULL, NULL, req->path);

I only know it was the last error (after shutdown, after_write, on_connection)

luv_io_error(L, uv_last_error(uv_default_loop()).code, NULL, NULL, NULL);