Include nulls in GET /notebooks/:id

Always include the error fields, but leave them as null if not set. This
makes the errors slightly more predictable to find and observe.

src/noteburst/handlers/v1/handlers.py

@@ -82,7 +82,7 @@ async def post_nbexec(
     "/notebooks/{job_id}",
     summary="Get information about a notebook execution job",
     response_model=NotebookResponse,
-    response_model_exclude_none=True,
+    response_model_exclude_none=False,
     responses={404: {"description": "Not found", "model": ErrorModel}},
 )
 async def get_nbexec_job(