[bug] raise when real dataset has not enough examples in evaluation

[anth2o]

This PR fixes a bug in the evaluation. If the real dataset has not enough samples in the test set, an exception is raised.

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[Marvin182]

Thanks a lot.
Can we use tf.errors.OutOfRangeError instead of defining a new exception?

[anth2o]

tf.errors.OutOfRangeError needs 'node_def', 'op', and 'message' to be raised, so I decided to raise a custom Exception which was more explicit. But if you prefer, I can raise the original exception.