Async before pipeline exception fix

An async `BeforePipeLineItem` that threw an exception never completed. 

This resulted in a neverendingrequest:
```c#
public class AsyncBeforeModule : NancyModule
{
    public AsyncBeforeModule()
    {
        Before.AddItemToStartOfPipeline(async (ctx, c) =>
        {
            await Task.Delay(1000);
            throw new Exception("aaargg!!");
        });

        Get["/"] = _ => "Hoi";
    }
}
```