Identify httpx ReadTimeout errors from execution

Specifically identify when the JupyterLab /execution request fails
because of a timeout.

src/noteburst/jupyterclient/jupyterlab.py

@@ -732,13 +732,22 @@ async def execute_notebook(
                 content=json.dumps(notebook).encode("utf-8"),
                 headers=headers,
             )
+        except httpx.ReadTimeout as e:
+            raise JupyterError(
+                url=exec_url,
+                username=self.user.username,
+                status=r.status_code,
+                reason="/execution endpoint timeout",
+                method="POST",
+                body=str(e),
+            ) from e
         except httpx.HTTPError as e:
             # This often occurs from timeouts, so we want to convert the
             # generic HTTPError to a JupyterError.
             raise JupyterError(
                 url=exec_url,
                 username=self.user.username,
-                status=500,
+                status=r.status_code,
                 reason="Internal Server Error",
                 method="POST",
                 body=str(e),