Minor improvement to test failure message when a best match test fails.

Author: Julian

Diff for: jsonschema/tests/test_exceptions.py

@@ -8,8 +8,12 @@
 class TestBestMatch(TestCase):
     def best_match_of(self, instance, schema):
         errors = list(_LATEST_VERSION(schema).iter_errors(instance))
+        msg =  f"No errors found for {instance} under {schema!r}!"
+        self.assertTrue(errors, msg=msg)
+
         best = exceptions.best_match(iter(errors))
         reversed_best = exceptions.best_match(reversed(errors))
+
         self.assertEqual(
             best._contents(),
             reversed_best._contents(),