feat: Adds support for errors.py to also use 'errors' for error_details  (#1281)

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [x] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/google-api-python-client/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [x] Ensure the tests and linter pass
- [x] Code coverage does not decrease (if any source code was changed)
- [x] Appropriate docs were updated (if necessary)

Fixes #1279 🦕
diff --git a/tests/test_errors.py b/tests/test_errors.py
index 9c139a6..955526f 100644
--- a/tests/test_errors.py
+++ b/tests/test_errors.py
@@ -141,5 +141,6 @@
             reason="Failed",
         )
         error = HttpError(resp, content)
-        self.assertEqual(str(error), '<HttpError 400 when requesting None returned "country is required". Details: "country is required">')
-        self.assertEqual(error.error_details, 'country is required')
+        expected_error_details = "[{'domain': 'global', 'reason': 'required', 'message': 'country is required', 'locationType': 'parameter', 'location': 'country'}]"
+        self.assertEqual(str(error), '<HttpError 400 when requesting None returned "country is required". Details: "%s">' % expected_error_details)
+        self.assertEqual(str(error.error_details), expected_error_details)