[entities] use try/except to get or create CeleryTask

IMHO this looks better than "if rset".
Also log created entity eid.

entities.py

@@ -273,15 +273,15 @@ class CeleryTaskAdapter(ICeleryTask):
 
         for asr in tree(result):
             task_id = six.text_type(asr.id)
-            rset = self._cw.find('CeleryTask', task_id=task_id)
-            if rset:
-                cwtask = rset.one()
-            else:
-                self.info("create a CeleryTask for %s" % task_id)
+            try:
+                cwtask = self._cw.find('CeleryTask', task_id=task_id).one()
+            except NoResultError:
                 cwtask = self._cw.create_entity(
                     'CeleryTask',
                     task_name=UNKNOWN_TASK_NAME,
                     task_id=six.text_type(task_id))
+                self.info("Create <CeleryTask %s (task_id %s)>",
+                          cwtask.eid, task_id)
             if not cwtask.parent_task and self.entity is not cwtask:
                 self.info('Set %s parent_task to %s (%s)', cwtask.task_id,
                           self.entity, self.entity.task_id)