implements xstatus in graph context, based on node xstatus.

bonobo/execution/contexts/graph.py

@@ -148,3 +148,11 @@ class GraphExecutionContext(BaseContext):
     def unregister_plugins(self):
         for plugin_context in self.plugins:
             plugin_context.unregister()
+
+
+    @property
+    def xstatus(self):
+        """
+        UNIX-like exit status, only coherent if the context has stopped.
+        """
+        return max(node.xstatus for node in self.nodes)