prevent error override, fix traceback type (#788)

* prevent error override, fix traceback type

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

papermill/execute.py

@@ -215,14 +215,18 @@ def raise_for_execution_errors(nb, output_path):
                     break
 
         # handle the CellExecutionError exceptions raised that didn't produce a cell error output
-        if not has_sys_exit and cell.get("metadata", {}).get("papermill", {}).get("exception") is True:
+        if (
+            error is None
+            and not has_sys_exit
+            and cell.get("metadata", {}).get("papermill", {}).get("exception") is True
+        ):
             error = PapermillExecutionError(
                 cell_index=index,
                 exec_count=cell.execution_count,
                 source=cell.source,
                 ename="CellExecutionError",
                 evalue="",
-                traceback="",
+                traceback=[],
             )
             break