better errors

Instead of throwing a string error, create proper errors: https://github.com/praekelt/vumi-jssandbox-toolkit/blob/master/lib/interaction_machine.js#L44
It would be nice to assert that the error has the right message by using the custom error validation of ``assert.throws`` (http://nodejs.org/api/assert.html#assert_assert_throws_block_error_message).