Adds __isset magic method need for Twig use.

Was trying to use your lib in a Synfony2/Twig environment and ran into the problem of  not being able to use the message property of the Braintree_Error_Validation class.

Found this documentation http://twig.sensiolabs.org/doc/recipes.html#using-dynamic-object-properties that explains that to use the __get in a Twig environment you also need the __isset method.

Thanks.