create a new exception ZoneRequestError to use for returning errors when zone requests couldn't complete

nova/scheduler/api.py

@@ -168,7 +168,10 @@ def child_zone_helper(zone_list, func):
             url = zone.api_url
             LOG.warn(_("Failed request to zone; URL=%(url)s: %(e)s")
                     % locals())
-            return e
+            # This is being returned instead of raised, so that when results are
+            # processed in unmarshal_result() after the greenpool.imap completes,
+            # the exception can be raised there if no other zones had a response.
+            return exception.ZoneRequestError()
         else:
             return func(nova, zone)