Specific exception for stale cluster state was added.

Specific exception should be thrown when cluster state
is stale. Current usage is to raise this exception if
compute_model.state is True.
Bug was describeid by Jean-Emile DARTOIS.

Change-Id: Iaddb4cc8007c51bb14759c9da829751e834499d0
Closes-Bug: #1621855

watcher/common/exception.py

@@ -340,6 +340,10 @@ class MetricCollectorNotDefined(WatcherException):
     msg_fmt = _("The metrics resource collector is not defined")
 
 
+class ClusterStateStale(WatcherException):
+    msg_fmt = _("The cluster state is stale")
+
+
 class ClusterDataModelCollectionError(WatcherException):
     msg_fmt = _("The cluster data model '%(cdm)s' could not be built")