Merge pull request #150 from vit-/fix/plugin-execution-context-shutdown

Check if PluginExecutionContext was started before shutting it down.

bonobo/execution/plugin.py

@@ -16,6 +16,7 @@ class PluginExecutionContext(LoopingExecutionContext):
             self.wrapped.initialize()
 
     def shutdown(self):
+        if self.started:
             with recoverable(self.handle_error):
                 self.wrapped.finalize()
         self.alive = False