Async filter exception bug

If we have an async response (returning CompletionStage for example, and a filter throws an exception while writing the response, the exception used to be silently ignored by CompletionStage, which meant that the client would hang waiting for the response, and nothing would be logged.

I fixed this, but it also means I don't throw UnhandledException to the container anymore in such cases, because it's very hard (if not impossible) to know at that point if the container is still in our call stack, or if we're in another thread entirely, in which case the container would never get the exception, and the exception would be caught by whatever happened to complete the async response, which is just wrong.