No bug: [taskgraph] Improve the error message in Graph.transitive_closure; r=mhentges

Differential Revision: https://phabricator.services.mozilla.com/D48000

--HG--
extra : moz-landing-system : lando

taskcluster/taskgraph/graph.py

@@ -50,7 +50,8 @@ class Graph(object):
         transitive_closure([b], reverse=True).nodes == set([b, c, d])
         """
         assert isinstance(nodes, set)
-        assert nodes <= self.nodes
+        if not (nodes <= self.nodes):
+            raise Exception("Unknown nodes in transitive closure: {}".format(nodes - self.nodes))
 
         # generate a new graph by expanding along edges until reaching a fixed
         # point