fix errorTarget with websockets

[minrk]

• don't build standard error responses for websocket errors, just send the error code and empty response. Websockets can't access error responses anyway.
• make sure to resume error request if incoming response is already over, otherwise we'll get leftover sockets (this is why we kept accumulating ESTABLISHED sockets - we never consumed the error request because the incoming response had already been closed by that point, and you need to resume() to consume a response if you aren't going to use it.

for more information, see #557

[consideRatio]

Amazing work tracking the details of this down @minrk!!!

This looks good to me!