[test] Skip `errors` tests if there is not any

test/test_chordpro/test_parser.py

@@ -91,6 +91,7 @@ class FileTest(unittest.TestCase, metaclass=dynamic.DynamicTest):
                         cls._create_test(base, in_format, out_format),
                         )
 
+            if os.path.isdir("errors"):
                 with cls.chdir('errors'):
                     for source in sorted(glob.glob('*.*.source')):
                         [*base, in_format, _] = source.split('.')