reformat_slices: Fix slice/length comparison.

Add a proper `ValueError` if the `slice`s and lengths don't match
one-to-one. Also include a test to make sure this exception is properly
raised.

kenjutsu/kenjutsu.py

@@ -166,7 +166,8 @@ def reformat_slices(slices, lengths=None):
     except TypeError:
         new_lengths = (new_lengths,)
 
-    assert (len(new_slices) == len(new_lengths))
+    if len(new_slices) != len(new_lengths):
+        raise ValueError("There must be an equal number of slices to lengths.")
 
     new_slices = list(new_slices)
     for i, each_length in enumerate(new_lengths):

tests/test_kenjutsu.py

@@ -82,6 +82,14 @@ def test_reformat_slice(self):
 
 
     def test_reformat_slices(self):
+        with self.assertRaises(ValueError) as e:
+            kenjutsu.reformat_slices((slice(None),), (1, 2))
+
+        self.assertEqual(
+            str(e.exception),
+            "There must be an equal number of slices to lengths."
+        )
+
         rf_slice = kenjutsu.reformat_slices(slice(None))
         self.assertEqual(
             rf_slice,