🐛 Properly raise RepoNotFoundError when not authenticated (#17651)

* Raise RepoNotFoundError in case of 401

* Include changes from revert-17646-skip_repo_not_found

* Add a comment

* 💄 Code quality

* 💚 Update `get_from_cache` test

* 💚 Code quality & skip failing test

tests/models/auto/test_modeling_flax_auto.py

@@ -77,7 +77,6 @@ class FlaxAutoModelTest(unittest.TestCase):
 
             eval(**tokens).block_until_ready()
 
-    @unittest.skip("Temp bug in the Hub not returning RepoNotFound errors.")
     def test_repo_not_found(self):
         with self.assertRaisesRegex(
             EnvironmentError, "bert-base is not a local folder and is not a valid model identifier"