message: add msg check
diff: @@ -1231,7 +1231,7 @@ class PolyvalTest(test.TestCase):
  def test_coeffs_raise(self):
  x = np.random.rand(2, 2).astype(np.float32)
  coeffs = {}
  - with self.assertRaises(ValueError):
  + with self.assertRaisesRegexp(ValueError, "Argument coeffs must be list"):
  math_ops.polyval(coeffs, x)
  