Gracefully fail if a Listener fails to start

[halter73]

If a Listener throws from CreateListenSocket (because the port is already in use for example), then a NullReferenceException is thrown from the Listener's dispose method that hides the original exception.

Example error:

C:\kestrel\CoreCLRMulti>.\kestrel
System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.AspNet.Server.Kestrel.Http.Listener`1.<>c__DisplayClass10_0.<Dispose>b__0(Object _)

Unhandled Exception: System.ObjectDisposedException: Safe handle has been closed
   at System.Runtime.InteropServices.SafeHandle.DangerousAddRef(Boolean& success)
   at System.StubHelpers.StubHelpers.SafeHandleAddRef(SafeHandle pHandle, Boolean& success)
   at Microsoft.AspNet.Server.Kestrel.Networking.Libuv.async_send(UvAsyncHandle handle) 
...