Neural Monkey should throw Exception when tf.Saver.restore fails

[varisd]

Try-catch block at
https://github.com/ufal/neuralmonkey/blob/master/neuralmonkey/learning_utils.py#L250

Print a warning, when tf.Saver.restore while ignoring the caught exception. This leads to a confusing behavior where user might think that the variables present in the checkpoint were correctly loaded. This is not true, since the variable loading is halted when the exception is thrown.