Fix exception handling of async actions of services

The continuation Task in ServiceStackHandlerBase.HandleResponse()
examines the Task state in wrong order. Checks task.IsCompleted first,
which is "true when the task is in one of the three final states:
RanToCompletion, Faulted, or Canceled."
(http://msdn.microsoft.com/en-us/library/system.threading.tasks.task.iscompleted)

Therefore the errorCallback() never gets called, defeating the Error Handling
mechanism when a service returns asynchronous Tasks that raise Exceptions.

The fix I suggest is not tested, although seems to be trivial.
Merge pull request #880 from robert4/patch-1

Fix exception handling of async actions of services