🐛 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

examples/research_projects/lxmert/demo.ipynb

@@ -6,7 +6,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "#%pip install-r requirements.txt"
+    "# %pip install-r requirements.txt"
    ]
   },
   {

examples/research_projects/visual_bert/demo.ipynb

@@ -4,7 +4,7 @@
    "cell_type": "code",
    "execution_count": 1,
    "source": [
-    "#%pip install-r requirements.txt"
+    "# %pip install-r requirements.txt"
    ],
    "outputs": [],
    "metadata": {}