Add error message for predicting with untrained PipeOp or Graph

[advieser]

closes #893

[advieser]

Added error message for Graph would not be strictly necessary, I think, because PipeOps and Learners would throw error. However, error message makes more sense this way.

I have not added something similar to GraphLearner, as it inherits from Learner which already has an appropriate error message. Only slight discrepancy is that the error message says Learner, and not GraphLearner, in that case.