Throw exception in CorpusReader.train_batch_gen() if no training utte…

…rances are found.

persephone/corpus_reader.py

@@ -124,9 +124,14 @@ def make_batches(self, utterance_fns: Sequence[Path]) -> List[Sequence[Path]]:
     def train_batch_gen(self) -> Iterator:
         """ Returns a generator that outputs batches in the training data."""
 
+        if len(self.train_fns) == 0:
+            raise PersephoneException("No training data available; cannot
+                                       generate training batches.")
+
         # Create batches of batch_size and shuffle them.
         fn_batches = self.make_batches(self.train_fns)
 
+
         if self.rand:
             random.shuffle(fn_batches)