[native] Make IntoPy conversion fallible (#639)

* Make IntoPy fallible
* Simplify test case so it works on 3.6

libcst/_parser/tests/test_parse_errors.py

@@ -6,8 +6,10 @@
 
 from textwrap import dedent
 from typing import Callable
+from unittest.mock import patch
 
 import libcst as cst
+from libcst._nodes.base import CSTValidationError
 from libcst._parser.entrypoints import is_native
 from libcst.testing.utils import data_provider, UnitTest
 
@@ -172,3 +174,10 @@ def test_parser_syntax_error_str(
             parse_fn()
         if not is_native():
             self.assertEqual(str(cm.exception), expected)
+
+    def test_native_fallible_into_py(self) -> None:
+        with patch("libcst._nodes.expression.Name._validate") as await_validate:
+            await_validate.side_effect = CSTValidationError("validate is broken")
+            with self.assertRaises(Exception) as e:
+                cst.parse_module("foo")
+            self.assertIsInstance(e.exception, (SyntaxError, cst.ParserSyntaxError))