How to raise exception in model?

I have some custom functions in model classes which processes some data, and add custom attribute to the model. Thing is, I do not know how could I raise an exception if this functions that are accessed when the template is generated (the error seems to be only silent, so it will process the template further but gives NO ERROR )

In the template

    {{ entry.state }}

In the model:

    @property

    def state(self):

        somedict = {'a': 111}

        try:

           print somedict['b']

        except Exception as e:

           FATAL_ERROR

What should I put in the place of fatal_error so that the template processing should stop immediatelly, or gives some exception to the render function ? 

Thanks

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