Log exception downloading artifacts stack (#1260)

Author: dmose

bot/code_coverage_bot/artifacts.py

@@ -182,6 +182,8 @@ def download_all(self) -> None:
                 exc = future.exception()
                 if exc is not None:
                     logger.error("Exception while downloading artifacts", exception=exc)
+                    from traceback import format_exception
+                    logger.error(format_exception(exc, exc, exc.__traceback__))
                     for f in futures:
                         f.cancel()