Finish all edge cases...

app.py

@@ -1,5 +1,7 @@
 import os
 import httpx
+import nbformat
+from nbformat import NotebookNode, ValidationError
 from nbconvert import HTMLExporter
 from starlette.applications import Starlette
 from starlette.exceptions import HTTPException
@@ -29,7 +31,31 @@ async def convert(req: Request):
         follow_redirects=True,
         # httpx no follow redirect by default
     )
-    print(r.text)
+    if r.status_code != 200:
+        raise HTTPException(
+            400, f"Got an error {r.status_code} when fetching remote file"
+        )
+
+    # Capture potential validation error:
+    try:
+        notebook_node: NotebookNode = nbformat.reads(
+            r.text,
+            as_version=4,
+        )
+    except nbformat.reader.NotJSONError:
+        print(400, f"Notebook is not JSON ({url})")
+        raise HTTPException(400, f"Notebook is not JSON ({url})")
+    except ValidationError as e:
+        print(
+            400,
+            f"Notebook is invalid according to nbformat: {e} ({url})",
+        )
+        raise HTTPException(
+            400,
+            f"Notebook is invalid according to nbformat: {e} ({url})",
+        )
+
+    print(f"Input: nbformat v{notebook_node.nbformat}.{notebook_node.nbformat_minor}")
     return HTMLResponse("<strong>FOO</strong>")