Wake JS on libuv poll errors instead of dropping the callback

[nigrosimone]

on_io_readable and on_io_writable look only at revents. But libuv signals a poll error with
status < 0 and revents == 0, and before calling the callback it already runs uv__io_stop and
uv__handle_stop, so the fd is out of the loop and no other event can arrive.

The result is that a connection reset by the peer becomes silent while it still looks alive.

I found it with pipelined queries in pg-native, where losing a backend in the middle of a batch left
the client with a connection that never completes and never fails.