Add a message for test_check_nullbytes

Author: florentx

testsuite/test_api.py

@@ -330,7 +330,8 @@ def test_check_nullbytes(self):
         pep8style = pep8.StyleGuide()
         count_errors = pep8style.input_file('stdin', lines=['\x00\n'])
 
-        self.assertTrue(sys.stdout[0].startswith("stdin:1:1: E901 TypeError"))
+        self.assertTrue(sys.stdout[0].startswith("stdin:1:1: E901 TypeError"),
+                        msg='Output is %r' % sys.stdout[0])
         self.assertFalse(sys.stderr)
         self.assertEqual(count_errors, 1)