title: Error on async job
body: <p>I'm trying to create an async task that will not block the request. The user make the request, the task will start, and the controller will render "Job is running...", this is for avoid the request being blocked waiting the task to complete.
  Once the task is finish, it will execute the onComplete and do something with the result of that task (for example call a service that will send a mail to an user)</p>
  
  <pre><code>| Error 2014-09-16 17:38:56,721 [Actor Thread 3] ERROR gpars.LoggingPoolFactory  - Async execution error: null
  </code></pre>
  
  <p>The code is the following:</p>
  
  <pre><code>package testasync
  
  import static grails.async.Promises.*
  
  class TestController {
  
    def index() {
      //Create the job
      def job1 = task {
          println 'Waiting 10 seconds'
          Thread.sleep(10000)
          return 'Im done'
      }
      //On error
      job1.onError { Throwable err -&gt;
          println "An error occured ${err.message}"
      }
      //On success
      job1.onComplete { result -&gt;
          println "Promise returned $result"
      }
      render 'Job is running...'
    }
  </code></pre>
  
  <p>Complete stacktrace:</p>
  
  <pre><code>| Error 2014-09-17 10:35:24,522 [Actor Thread 3] ERROR gpars.LoggingPoolFactory  -  Async execution error: null
  Message: null
     Line | Method
   -&gt;&gt;   72 | doCall    in org.grails.async.factory.gpars.GparsPromise$_onError_closure2
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
    |     62 | run       in groovyx.gpars.dataflow.DataCallback$1
    |   1145 | runWorker in java.util.concurrent.ThreadPoolExecutor
    |    615 | run       in java.util.concurrent.ThreadPoolExecutor$Worker
    ^    745 | run . . . in java.lang.Thread
  </code></pre>